Control of a limited slip differential based on a steering angle of a vehicle

ABSTRACT

A limited slip differential (LSD) transfers a torque from an engine to left and right wheels of a vehicle. A current angle of a steering device is determined. Rotational speeds of 5 the left and right driven wheels are also determined. A current wheel slip of is calculated as a difference between the rotational speeds of the left and right wheels. Maximum and minimum allowed wheel slips are calculated as a function of the current steering angle. The LSD is controlled so that the current wheel slip stays in a range between the maximum and minimum allowed wheel slips. To this end, the LSD is loaded if the wheel slip falls outside of the range 0 between the maximum and minimum allowed wheel slips.

CROSS-REFERENCE

The present application claims priority from U.S. Provisional PatentApplication No. 62/560,591, filed on Sep. 19, 2017 and from UnitedStates Provisional Patent Application No. 62/586,427, filed on Nov. 15,2017, the entirety of which being incorporated herein by reference.

FIELD OF TECHNOLOGY

The present technology relates to a control of a limited slipdifferential based on a steering angle of a vehicle, to a method ofcontrolling a limited slip differential, and to a vehicle including thelimited slip differential.

BACKGROUND

There exist various types of vehicles used mainly in off-roadconditions. One such type is the side-by-side off-road vehicle. The name“side-by-side” refers to the seating arrangement of the vehicle in whichthe driver and a passenger are seated side-by-side. Some side-by-sideoff-road vehicles also have a second row of seats to accommodate one ormore additional passengers. These vehicles typically have an opencockpit, a roll cage and a steering wheel.

To be able to operate in off-road conditions, a side-by-side off-roadvehicle needs to be able to handle bumpy terrain and to operate onvarious surfaces including, but not limited to, sand, dirt and mud.These conditions represent unique challenges not typically encounteredwhen designing on-road vehicles such as cars. One such challenge lies inthe provision of torque to each driving wheel under various conditionssuch as amount of steering, vehicle orientation when climbing a hill,rate of acceleration, slippery or rocky terrain, and the like.

A differential is commonly used to receive torque from a driving shaftand to redirect the torque via half-shafts toward two driving wheels ofthe vehicle. The differential allows the half-shafts and correspondingwheels to rotate at distinct rates, as it is desirable to allow theinside wheel to rotate at a somewhat lower rate than the outside wheelwhen the vehicle is in a turn. However, when one of the wheels is onslippery terrain, the differential may direct all torque on that onewheel, which may spin unnecessarily without allowing the vehicle tomove, no torque being delivered on the other wheel. A limited slipdifferential (LSD) is conventionally used to limit the rotational speeddifference between left and right driven wheels of a wheel set. In avehicle equipped with a LSD, should the left wheel (for example) be on apatch of low friction terrain, it will only spin to a small extentbefore the LSD starts transmitting torque on the opposite right wheel.As the right wheel may be on terrain providing better traction, thisallows the vehicle to move until both wheels are on terrain providingbetter traction.

Conventional LSDs suffer from a number of operational limitations.

A conventional LSD may lock both wheels of the wheel set as soon asthere is some rotational speed difference between the two wheels. Thismay cause the LSD to lock both wheels when the vehicle is in a curveeven though both wheels may have good traction at the time. Whilelocking the LSD may prevent wheel spin, steering of the vehicle becomesdifficult when the LSD is locked because a locked LSD acts counter tonatural speed differences between the slower wheel on the inside of acurve and the faster wheel on the outside of the curve.

To prevent locking of the LSD during every turn of the vehicle, the LSDmay be configured to allow a fairly large rotational speed differencebetween the two wheels of the axis. While this design may preventunnecessary locking of the LSD at every turn, it may delay the transferof torque to the wheel having better traction when the opposite wheel ison slippery terrain. Such delays in the locking of the LSD may renderthe vehicle difficult to control on slippery terrain and lead to anegative driver experience. This lack of proactivity of the conventionalLSD may even cause the vehicle to remain stuck on low friction terrain,such as when on mud or ice, or lose momentum when climbing on rockyterrain. Moreover, delays in the locking of the LSD may cause importantspinning of the driven wheels upon heavy acceleration from a standingstart.

When a vehicle is travelling in deep mud or in similar slippery drivingconditions, the torque being applied to the wheels may change frequentlyand may change by a large amount. This could cause the LSD to constantlylock and unlock again. This behavior of the LSD is not only inefficient,but may be detrimental to the driving experience while potentiallycausing premature damage to the LSD. Some LSDs have a manual lockingmode that may be used to overcome this constant locking and unlockingproblem. The user of the vehicle may manually lock the LSD, for examplewhen the user predicts that the vehicle is about to encounter a mudpatch. The LSD remains locked until unlocked by the user. While this mayhelp preventing that the vehicle becomes stuck in the mud patch, it mayrender the vehicle difficult to drive if the LSD is still manuallylocked when better surface conditions are met again, steering becomingdifficult for example. In some off-road paths, the user might need tofrequently lock and unlock again the LSD. The vehicle may remain stuckin a mud patch if the user does not react in good time to manually lockthe LSD when slippery driving conditions are met.

There is therefore a desire for a control of limited slip differentialsthat addresses the above issues.

SUMMARY

It is an object of the present technology to ameliorate at least some ofthe inconveniences present in the prior art.

The present technology provides a limited slip differential (LSD)controlled according to a steering angle of a vehicle. The LSD iscontrolled to maintain within a predetermined range a relative speeddifference of two driven wheels connected to the LSD, this differencebeing called a wheel slip. In the context of the present disclosure, thewheel slip may for example be calculated as the speed of the left wheelminus the speed of the right wheel. The wheel slip is positive when theleft wheel rotates faster than the right wheel and negative when theleft wheel rotates at a slower rate than the right wheel. The wheel slipmay as well be calculated as the speed of the right wheel minus thespeed of the left wheel. The present technology relates to the relativespeed difference between the two driven wheels. A current angle of asteering device is determined. Rotational speeds of the left and rightdriven wheels are also determined. A maximum allowed wheel slip and aminimum allowed wheel slip are calculated as a function of the currentsteering angle. A current wheel slip is calculated as a differencebetween the rotational speeds of the left and right driven wheels. TheLSD is controlled so that the current wheel slip remains within a rangedefined by the maximum and minimum allowed wheel slips. The maximum andminimum allowed wheel slips may be calculated according to a slip marginand according to an expected wheel slip that, in turn, is calculatedbased on the current steering angle, on a configuration of the vehicleas defined by a slip ratio, and on a rotational speed of the wheelsconnected to the LSD.

According to one aspect of the present technology, there is provided amethod of controlling a limited slip differential (LSD) of a vehicle,the vehicle having a steering device, the LSD, and left and right drivenwheels operably connected to the LSD. The method comprises: determiningrotational speeds of the left and right driven wheels; calculating acurrent wheel slip as a rotational speed difference between one of theleft and right driven wheels and the other one of the left and rightdriven wheels; determining a current angle of the steering device;calculating a maximum allowed wheel slip and a minimum allowed wheelslip as a function of the current angle of the steering device;detecting that the current wheel slip is outside a range between themaximum and minimum allowed wheel slips; and in response to detectingthat the current wheel slip is outside the range between the maximum andminimum allowed wheel slips, loading the LSD.

In some implementations of the present technology, calculating themaximum allowed wheel slip and the minimum allowed wheel slip as afunction of the current angle of the steering device comprises:determining an average of the rotational speeds of the left and rightdriven wheels; calculating an expected wheel slip based on: a ratiobetween the current angle of the steering device and a maximum steeringangle, the average of the rotational speeds of the left and right drivenwheels, and a slip ratio for the vehicle; while the LSD is not loaded,calculating the maximum allowed wheel slip by adding a slip margin tothe expected wheel slip; and while the LSD is not loaded, calculatingthe minimum allowed wheel slip by subtracting the slip margin from theexpected wheel slip.

In some implementations of the present technology, the slip margin isselected based on the average of the rotational speeds of the left andright driven wheels.

In some implementations of the present technology, the method furthercomprises, while the LSD is loaded: calculating the maximum allowedwheel slip as the greater of: the slip margin, and a sum of the expectedwheel slip and the slip margin; and calculating the minimum allowedwheel slip as the lesser of: zero minus the slip margin, and theexpected wheel slip minus the slip margin.

In some implementations of the present technology, a loading level ofthe LSD is calculated based on a magnitude of an excess of the currentwheel slip in relation to the range between the maximum and minimumallowed wheel slips.

In some implementations of the present technology, the method furthercomprises, after loading the LSD in response to detecting that thecurrent wheel slip is outside the range between the maximum and minimumallowed wheel slips: detecting that the current wheel slip has returnedwithin the range between the maximum and minimum allowed wheel slips;and in response to detecting that the current wheel slip has returnedwithin the range between the maximum and minimum allowed wheel slips,unloading the LSD.

In some implementations of the present technology, unloading the LSDcomprises gradually unloading the LSD.

In some implementations of the present technology, loading the LSDcomprises compressing a clutch of the LSD to reduce the rotational speeddifference between the left and right driven wheels.

In some implementations of the present technology, compressing theclutch of the LSD comprises using an electric motor to drive a gear setand a ball ramp to apply a torque on the clutch.

In some implementations of the present technology, compressing theclutch of the LSD further comprises using a solenoid to lock the gearset.

In some implementations of the present technology, the method furthercomprises, in response to detecting that the current wheel slip isoutside the range between the maximum and minimum allowed wheel slips,calculating an initial torque for application to the clutch; followingthe application of the initial torque on the clutch: increasing thetorque applied on the clutch if the current wheel slip is still outsidethe range between the maximum and minimum allowed wheel slips,decreasing the torque applied on the clutch if the current wheel slipfalls within the range between the maximum and minimum allowed wheelslips, and terminating the loading to the LSD when the torque applied onthe clutch falls to zero.

According to another aspect of the present technology, there is provideda differential assembly for use in a vehicle having a steering deviceand left and right driven wheels. The differential assembly comprises: alimited slip differential (LSD) operatively connectable to a driveshaftand to the left and right driven wheels, the LSD being adapted fortransferring torque from the driveshaft to the left and right drivenwheels; a steering angle sensor operatively connectable to the steeringdevice; wheel speed sensors operatively connectable to the left andright driven wheels; and a control unit operatively connected to theLSD, to the steering angle sensor and to the wheel speed sensors, thecontrol unit being adapted for: calculating, based on measurements ofrotational speeds of the left and right driven wheels received from thewheel speed sensors, a current wheel slip as a rotational speeddifference between one of the left and right driven wheels and the otherone of the left and right driven wheels, calculating, based on ameasurement received from the steering angle sensor, a maximum allowedwheel slip and a minimum allowed wheel slip as a function of a currentangle of the steering device; detect that the current wheel slip isoutside a range between the maximum and minimum allowed wheel slips, andin response to detecting that the current wheel slip is outside therange between the maximum and minimum allowed wheel slips, controllingloading of the LSD.

In some implementations of the present technology, the differentialassembly further comprises an electric motor, the control unitcommanding the electric motor to load the LSD.

In some implementations of the present technology, the LSD furthercomprises a compressible clutch, the electric motor loading the LSD byapplying a torque on the clutch.

In some implementations of the present technology, the LSD furthercomprises a gear set and a ball ramp, the gear set being adapted fortransferring the torque from the electric motor to the ball ramp forcompressing the clutch.

In some implementations of the present technology, the differentialassembly further comprises a solenoid having a tooth adapted forengaging the gear set when the solenoid is energized, the control unitbeing further adapted for controlling the solenoid for locking the LSD.

In some implementations of the present technology, the control unit isfurther adapted for controlling the LSD to apply a maximum load beforelocking the LSD.

In some implementations of the present technology, the control unitcomprises: an input port adapted for receiving measurements from thesteering angle sensor and from the wheel speed sensors; an output portadapted for forwarding control commands to the LSD; and a processoroperatively connected to the input port and to the output port, theprocessor being adapted for: calculating the maximum and minimum allowedwheel slips, calculating the current wheel slip, detecting that thecurrent wheel slip is outside the range between the maximum and minimumallowed wheel slips, and in response to detecting that the current wheelslip is outside the range between the maximum and minimum allowed wheelslips, causing the output port to forward a control command for loadingthe LSD.

In some implementations of the present technology, the control unitfurther comprises a memory storing configuration information forcontrolling the LSD; the processor is operatively connected to thememory; and the configuration information comprises: a maximum steeringangle for the vehicle, a slip ratio for the vehicle, and a slip mappingtable of relations between values of the slip margin speed values forthe left and right driven wheels.

In some implementations of the present technology, the processor isfurther adapted for: determining an average of the rotational speeds ofthe left and right driven wheels; calculating an expected wheel slipbased on: a ratio between the current angle of the steering device and amaximum steering angle, the average of the rotational speeds of the leftand right driven wheels, and a slip ratio for the vehicle; reading fromthe slip mapping table a slip margin for the average of the rotationalspeeds of the left and right driven wheels; while the LSD is not loaded,calculating the maximum allowed wheel slip by adding the slip margin tothe expected wheel slip; and while the LSD is not loaded, calculatingthe minimum allowed wheel slip by subtracting the slip margin from theexpected wheel slip.

In some implementations of the present technology, the processor isfurther adapted for, while the LSD is loaded: calculating the maximumallowed wheel slip as the greater of: the slip margin, and a sum of theexpected wheel slip and the slip margin; and calculating the minimumallowed wheel slip as the lesser of: zero minus the slip margin, and theexpected wheel slip minus the slip margin.

In some implementations of the present technology, the processor isfurther adapted for: calculating a loading level of the LSD based on amagnitude of an excess of the current wheel slip in relation to therange between the maximum and minimum allowed wheel slips; andconfiguring the control command to load the LSD according to thecalculated loading level.

In some implementations of the present technology, the processor isfurther adapted for, after causing the output port to forward thecontrol command for loading the LSD: causing the output port to forwarda control command to increase the load to the LSD if the current wheelslip is still outside the range between the maximum and minimum allowedwheel slips; and causing the output port to forward a control command todecrease the load to the LSD if the current wheel slip falls within therange between the maximum and minimum allowed wheel slips.

According to a further aspect of the present technology, there isprovided a vehicle, comprising: a frame; a front suspension assemblyconnected to the frame; a rear suspension assembly connected to theframe; a left driven wheel and a right driven wheel connected to one ofthe front and rear suspension assemblies; at least one other wheelconnected to an other one of the front and rear suspension assemblies; asteering device operatively connected to one of both driven wheels andthe at least one other wheel; an engine connected to the frame; atransmission operatively connected to the engine for receiving torquefrom the engine; a driveshaft operatively connected to the transmissionfor transferring torque from the transmission to the left and rightdriven wheels; and a differential assembly. The differential assemblycomprises: a limited slip differential (LSD) operatively connected tothe driveshaft and to the left and right driven wheels, the LSD beingadapted for transferring torque from the driveshaft to the left andright driven wheels; a steering angle sensor operatively connectable tothe steering device; wheel speed sensors operatively connectable to theleft and right driven wheels; and a control unit operatively connectedto the LSD, to the steering angle sensor and to the wheel speed sensors,the control unit being adapted for: calculating, based on measurementsof rotational speeds of the left and right driven wheels received fromthe wheel speed sensors, a current wheel slip as a rotational speeddifference between one of the left and right driven wheels and the otherone of the left and right driven wheels, calculating, based on ameasurement received from the steering angle sensor, a maximum allowedwheel slip and a minimum allowed wheel slip as a function of a currentangle of the steering device; detect that the current wheel slip isoutside a range between the maximum and minimum allowed wheel slips, andin response to detecting that the current wheel slip is outside therange between the maximum and minimum allowed wheel slips, controllingloading of the LSD.

In some implementations of the present technology, the vehicle furthercomprises: a transaxle for transferring torque from the transmission tothe at least one other wheel; and a selector adapted for selectivelyoperatively connecting the LSD to the driveshaft.

Implementations of the present technology each have at least one of theabove-mentioned object and/or aspects, but do not necessarily have allof them. It should be understood that some aspects of the presenttechnology that have resulted from attempting to attain theabove-mentioned object may not satisfy this object and/or may satisfyother objects not specifically recited herein.

Additional and/or alternative features, aspects and advantages ofimplementations of the present technology will become apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present technology, as well as otheraspects and further features thereof, reference is made to the followingdescription which is to be used in conjunction with the accompanyingdrawings, where:

FIG. 1 is a perspective view of an off-road vehicle taken from a front,left side;

FIG. 2 is a left side elevation view of the vehicle of FIG. 1;

FIG. 3 is a rear elevation view of an instrument panel of the vehicle ofFIG. 1;

FIG. 4 is a left side elevation view of a powertrain of the vehicle ofFIG. 1;

FIG. 5 is a left side cutaway view of the powertrain of FIG. 4;

FIG. 6 is a bottom plan view of the powertrain FIG. 4;

FIG. 7 is a perspective view, taken from a rear, left side, of a frontdifferential assembly of the powertrain of FIG. 4;

FIG. 8 is a schematic cross-sectional view of the differential assemblyof FIG. 7;

FIG. 9 is a cross-sectional view of an example construction of thedifferential assembly of FIG. 7;

FIG. 10 provides timing diagrams showing variations of a steering angle(top diagram), wheel slip variations and a range between maximum andminimum allowed wheel slips calculated by the engine control unit(middle diagram), and control commands for loading and/or locking theLSD (bottom diagram);

FIG. 11 is a graphical representation of a slip margin (top diagram)varying as a function of rotational speed of the front wheels of thevehicle of FIG. 1 (bottom diagram);

FIGS. 12a and 12b are a logic diagram showing operations of a method forcontrolling a limited slip differential based on a steering angle of avehicle;

FIG. 13 is a logic diagram showing details of a method of applying aload on the limited slip differential;

FIG. 14 is a block diagram of a control unit for the limited slipdifferential;

FIG. 15 is a block diagram showing internal operations of the controlunit for determining the predicted engine torque, according to animplementation;

FIG. 16 is a logic diagram showing operations of a method forcontrolling a limited slip differential based on an engine torque;

FIG. 17 is a block diagram showing internal operations of the controlunit for controlling the LSD in mud mode, according to animplementation;

FIG. 18 is a graph of an engine load line;

FIGS. 19a to 19e provide logic diagrams showing operations of a methodfor controlling a limited slip differential based on driving conditions;

FIG. 20 is a block diagram showing internal operations of the controlunit for controlling the LSD in trail active mode, according to animplementation;

FIG. 21 is a logic diagram showing operations of a method forcontrolling a limited slip differential based on an accelerator controlposition;

FIG. 22 is a logic diagram showing operations of a method forcontrolling a limited slip to stabilize the steering of a vehicle; and

FIG. 23 is a block diagram showing internal operations of the controlunit for determining the maximum and minimum allowed wheel according toan implementation.

DETAILED DESCRIPTION

Generally stated, the present technology provides control of a limitedslip differential (LSD) mounted on an axle of a vehicle, this controlbeing based at least in part on measurements provided by various sensorsto a control unit operatively connected to the LSD.

The present technology will be described with respect to a four-wheel,off-road vehicle having two side-by-side seats and a steering wheel.However, it is contemplated that at least some aspects of the presenttechnology may apply to other types of vehicles such as, but not limitedto, off-road vehicles having a handlebar and a straddle seat (i.e. anall-terrain vehicle (ATV)), off-road vehicles having more or less thanfour wheels, and on-road vehicles having three or more wheels and havingone or more seats.

Description of the Vehicle

The general features of the off-road vehicle 40 will be described withrespect to FIGS. 1, 2 and 3. The vehicle 40 has a frame 42, two frontwheels 44 connected to a front of the frame 42 by a front suspensionassembly 46 and two rear wheels 48 connected to the frame 42 by a rearsuspension assembly 50. Each one of the front and rear wheels 44, 48 hasa rim 45 and a tire 47. The rims 45 and tires 47 of the front wheels 44may differ in size from the rims and tires of the rear wheels 48. Inaddition, although four wheels 44, 48 are illustrated in the Figures,the vehicle 40 could include more or less than four wheels 44, 48.

The frame 42 defines a central cockpit area 52 inside which are disposeda driver seat 54 and a passenger seat 56. In the present implementation,the driver seat 54 is disposed on the left side of the vehicle 40 andthe passenger seat 56 is disposed on the right side of the vehicle 40.However, it is contemplated that the driver seat 54 could be disposed onthe right side of the vehicle 40 and that the passenger seat 56 could bedisposed on the left side of the vehicle 40. It is also contemplatedthat the vehicle 40 could include a single seat for the driver, or alarger number of seats, or a bench accommodating the driver and at leastone passenger. The driver operates the steering wheel 58 from the driverseat 54 to control an angle of the front wheels 44.

As can be seen in FIG. 2, an engine 62 is connected to the frame 42 in arear portion of the vehicle 40. The engine 62 is connected to acontinuously variable transmission (CVT) 64 disposed on a left side ofthe engine 62. The CVT 64 is operatively connected to a transaxle 66 totransmit torque from the engine 62 to the transaxle 66. The transaxle 66is disposed behind the engine 62. The transaxle 66 is operativelyconnected to the front and rear wheels 44, 48 to propel the vehicle 40.The engine 62, the CVT 64 and the transaxle 66 are supported by theframe 42. A variant of the vehicle 40 having another transmission typeis also contemplated.

The transaxle 66 is mechanically connected to a shifter 60 disposedlaterally between the two seats 54, 56. The shifter 60 allows the driverto select from a plurality of combinations of engagement of gears of thetransaxle 66, commonly referred to as gears. In the presentimplementation, the shifter 60 allows the driver to select between areverse gear, two forward gears (high and low) and a neutral position inwhich the transaxle 66 does not transmit torque to the wheels 44, 48. Itis contemplated that other types of connections between the shifter 60and the transaxle 66 could be used.

In an implementation, operative connection of the transaxle 66 to thefront wheels 44 is selectable, the selection being made using a drivemode selector provided in the vicinity of the driver. The drive modeselector may comprise a toggle switch 59 (FIG. 3) mounted on aninstrument panel 61 of the vehicle 40. The toggle switch 59 has two (2)positions for selecting a two-wheel mode or an all-wheel mode for thevehicle 40. The drive mode selector may also comprise a toggle switch 63having two (2) positions for manually locking and unlocking a limitedslip differential (shown on later Figures). The drive mode selector mayfurther comprise a toggle switch 65 having four (4) positions forselecting one of a normal mode, a trail active mode, a mud mode and arock crawling mode. It is contemplated that the toggle switch 65 mayonly permit selection of one or two of the trail active mode, the mudmode and the rock crawling mode in a vehicle that only has one or two ofthese modes available. Use of a rotary knob for selecting one of thevarious modes and use of distinct switches for turning on and off eachof the trail active mode, mud mode and rock crawling mode are alsocontemplated. It is also contemplated to the rotary knob or additionaltoggle switches may be used to select other modes, for example a sandmode, a snow mode, and the like.

Referring to FIGS. 4 to 6, the CVT 64 has a driving pulley 68 connectedto and driven by the engine 62 as well as a driven pulley 72 mounted tothe transaxle 66. A belt 76 transmits a torque imparted on the drivingpulley 64 by the engine 62 to the driven pulley 72 that in turntransmits the torque to the transaxle 66. The driving pulley 68 and thedriven pulley 72 permit a continuously variable transmission ratio byvirtue of the opening or closing of opposed conical side faces of one ormore of the pulleys. It should be understood that alternativetransmission configurations may be used.

In the vehicle 40, the transaxle 66 transmits the torque applied thereonby the driven pulley 72 to drive the rear wheels 48, when the drive modeselector is in a two-wheel mode, or to drive the front and rear wheels44, 48, when the drive mode selector in an all-wheel mode. The transaxleapplies a torque to the rear wheels 48 via corresponding half-shafts 78.To this end, the transaxle 66 includes a differential 80 operativelyconnected to the half shafts 78. Instead of the differential 80, use ofa spool gear is also contemplated. When the drive mode selector is inthe all-wheel mode, the transaxle 66 applies a portion of the torque onthe half shafts 78, and also applies another portion of the torque on afront driveshaft 82. A front end of the front driveshaft 82 is connectedto another driveshaft 84 via a universal joint 86. A front end of thedriveshaft 84 drives an input shaft 90 of a limited slip differential(LSD) 302 via another universal joint 92.

The LSD 302 is operatively connected to and drives left and right fronthalf-shafts 98. Laterally outward ends of the front half-shafts 98 areoperatively connected to and drive the front wheels 44.

Description of an Example of the Limited Slip Differential Assembly

FIGS. 7 and 8 show a limited slip differential assembly 300 includingthe LSD 302 connected to driven wheels of the vehicle 40. In animplementation, the differential assembly 300 drives the front wheels 44of the vehicle 40. It should be understood that the differentialassembly 300 could alternatively be used with the rear wheels 48 of thevehicle 40, or to any pair of wheels of any other type of vehicle. Thedifferential assembly 300 includes the LSD 302, a control unit 370, anactuator 372, a solenoid 382, and one or more sensors. The actuator 372,the solenoid 382 and the sensors are electrically connected to thecontrol unit 370. Sensors may include one or more wheel speed sensors376, 378, a vehicle speed sensor 380, a steering angle sensor 390, anaccelerator control sensor 392, an engine torque monitor 394, a shifterposition indicator 396, and a user command sensor 398. The user commandsensor 398 informs the control unit 370 of the state of the varioustoggle switches 59, 63 and 65. Some of these sensors may be present insome implementations and not present in some other implementations. Allof these sensors, when present, are communicatively coupled with thecontrol unit 370, to which they provide measurements and sensedinformation elements.

FIG. 8 illustrates a particular, non-limiting implementation of the LSD302. In the LSD 302 as shown on FIG. 8, the input shaft 90 is connectedto a first bevel gear acting as an input gear 304. Rotation of the inputshaft 90 causes a rotation of the input gear 304 that, in turn, causes arotation of a second bevel gear, or ring gear 306. Rotation of the ringgear 306 causes a rotation of first clutch plates 308 and of a carrier310. A shaft 312 connects the carrier 310 to a gear set that includes atleast two (2) planet gears 314, 316 and sun gears 318, 320. An outputshaft 322 is mounted to the sun gear 318, the shaft 322 and the sun gear318 rotating together. Likewise, an output shaft 324 is mounted to thesun gear 320, the shaft 324 and the sun gear 320 rotating together. Theoutput shafts 322 and 324 are operatively connected to the half shafts98 via universal joints or joints of other types (not shown) containedin boot covers 326 and 328 (shown on FIG. 7).

The LSD 302 has second clutch plates 330. When the clutch plates 308 and330 are not compressed, the LSD 302 is not loaded. The output shafts 322and 324 may rotate at the same speed or at distinct speeds. When bothoutput shafts 322 and 324 rotate at a same speed, they also both rotateat the same rate as the ring gear 306, the carrier 310 and the sun gears318, 320. At that time, the planet gears 314 and 316 do not rotate aboutthe axis of the shaft 312 (they only rotate about the axis of the ringgear 306, following the movement of the carrier 310). When the twooutput shafts 322 and 324 rotate at distinct speeds, a rotationaldifference of the sun gears 318 and 316 causes a rotation of the planetgears 314 and 316 about the axis of the shaft 312. In that case, torquefrom the input shaft 90 is unequally transferred to the output shafts322 and 324 and, ultimately, to the left and right wheels 44.

The actuator 372 may compress the clutch plates 308 and 330. Thiscompression reduces, and eventually eliminates, a rotational speeddifference between the ring gear 306 and the output shaft 324. If theclutch plates 308 and 330 are compressed to the point of eliminating anyrotational speed difference between the ring gear 306 and the outputshaft 324, the carrier 310 also rotates at the same speed as the outputshaft 324. The planetary gears 314 and 316 cannot turn about the axis ofthe shaft 312 so the sun gear 318 and the output shaft 322 also rotateat the same speed as the output shaft 324. The LSD 302 is theneffectively locked. In case of partial loading of the LSD 302, amoderate compression of the clutch plates 308 and 330 causes a reductionof a rotational speed difference between the ring gear 306 and theoutput shaft 324, without totally eliminating this difference. The LSD302 is at that time allowing a limited slip of the wheels 44.

The LSD 302 is a conventional clutch-type limited slip differential andis controllable to allow a predetermined maximum difference inrotational speeds between the left and right front wheels 44. It iscontemplated that any other suitable type of LSD 302 may alternativelybe used.

The LSD 302 is mechanically coupled to an actuator 372, for example anelectrical, hydraulic or magnetic actuator, that is electronicallycontrolled by a control unit 370. To regulate the difference inrotational speeds between the left and right front wheels 44, theactuator 372 can vary the compression on the clutch plates 308 and 330to vary the degree of engagement, or load, of the LSD 302. The LSD 302may be engaged, i.e. loaded, when the control unit 370 detects that oneof the wheels 44 is slipping.

In at least one implementation, in order to prevent eventual slipping ofthe wheels 44, the control unit 370 may control the LSD 302 to be loadedbefore the actual detection of a wheel slip. It can be said in such casethat the LSD 302 is preloaded. In the context of the present disclosure,differences between the terms “load” and “preload” primarily relate tothe circumstances under which the control unit 370 initiates the loadingof the LSD 302. The LSD 302 operates essentially in the same mannerwhether it is loaded or preloaded. Application of a preload to the LSD302 does not preclude further or increased loading of the LSD 302 in theevent of a wheel slip.

The control unit 370 may cause the LSD 302 to act as an opendifferential (fully disengaged), a locked differential (fully engaged),or at any intermediate degree of engagement. The control unit 370 iselectrically connected to wheel speed sensors 376, 378 that, on FIG. 8,are connected to the output shafts 324 and 322. The wheel speed sensors376, 378 may alternatively be connected to the front wheels 44, to thefront half-shafts 98, or to any other suitable component from which thecontrol unit 370 receives signals indicative of the rotational speeds ofthe left and right front wheels 44.

FIG. 9 is a cross-sectional view of an example construction of thedifferential assembly of FIG. 7. The actuator 372 comprises an electricmotor 288 that drives a gear set 276. A rotational motion of the gearset 276 is translated into an axial motion by a ball ramp 278. Thisaxial motion is used to apply a pressure generated by the electric motor288 to compress the clutch plates 308 and 330 of a clutch 374. Thiscompression of the clutch 374 loads the LSD 302 to reduce the relativeslip between the left and right half shafts 98. Sufficient compressionof the clutch 374 may effectively lock the LSD 302. However, even undermaximum compression, the clutch 374 may slip in some implementations,under severe conditions. Consequently, depending on the torque from theinput shaft 90 being applied to the LSD 302 and depending oncharacteristics of the clutch 374, the LSD 302 may not lock to anabsolute degree. In the context of the present disclosure, the LSD 302is considered locked when maximum torque is applied on the clutch 374although at the time a modest relative slip may still be present betweenthe left and right half shafts 98. Consequently, the “locking of the LSD302” should not be understood in the absolute.

In order to prevent overheating of the electric motor 288, a solenoid382 having a tooth 384 at its end may be energized so that the tooth 384meshes with a largest gear 386 of the gear set 276, thereby locking thegear set 276, the ball ramp 278 and the clutch 374 in a selected loadposition. As a result, the electric motor 288 no longer needs to beenergized to maintain the load to the LSD 302. In an implementation, thesolenoid 382 may be energized to lock the LSD 302 when the control unit370 determines that loading has been applied for at least apredetermined time period duration. It should be observed thatenergizing the solenoid 382 requires much less current than energizingthe electric motor 288. De-energizing the solenoid 382 causes it toretract, releasing the tooth 384 from the largest gear 386 of the gearset 276 and releasing the load to the LSD 302. In a variant, thesolenoid 382 may be configured so that its tooth 384 meshes with thelargest gear 386 of the gear set 276 when the solenoid 382 is notenergized, energizing the solenoid 382 thus causing a release of thegear set 276 and unlocking of the clutch 374.

In an implementation, maximum compression of the clutch 374 may beapplied by the electric motor 288, the gear set 276 and the ball ramp278 prior to energizing the solenoid 382. In the same or anotherimplementation, the solenoid 382 may also be energized to lock the LSD302 when a user manually activates the toggle switch 63 to select tolock the LSD 302, as indicated by the user command sensor 398 thatinforms the control unit 370 of a user request to lock the LSD 302. Insuch case, the user request to lock the LSD 302 may optionally cause amaximum load of the LSD 302 by maximum compression of the clutch 374 bythe electric motor 288, the gear set 276 and the ball ramp 278 prior toenergizing the solenoid 382.

Other implementations of the differential assembly 300 and of the LSD302 are also contemplated. The present technology is not limited to theparticular implementation illustrated on FIGS. 7 to 9. In particular, adifferential assembly that does not contain a ball ramp or clutch platesis also contemplated.

Control of the LSD 302 Based on a Steering Angle of the Vehicle 40

One aspect of the present technology provides control of the LSD 302connected to the driven wheels 44 of the vehicle 40 based at least inpart on rotational speeds of both left and right driven wheels 44 of thevehicle 40 and at least in part on a steering angle. In the context ofthe present disclosure, the steering angle may represent the angle of asteering wheel 58 or the angle of a handlebar, depending on the type ofsteering control mounted on the vehicle. In vehicles having so-calleddrive-by-wire steering systems, the ratio of a steering wheel input tothe angle of steered wheels may vary according to the speed of thevehicle and, in some cases, according to some other factors.

Considering that it is natural for the inside wheels 44 and 48 to rotateat a slower rate than the outside wheels 44 and 48 when the vehicle 40is in a curve, in an implementation, the control unit 370 determines anallowable slipping range between the left and right front wheels 44, theallowable slipping range being based at least in part on the steeringangle and on the speed of the vehicle 40. This allows the control unit370 to control loading of the LSD 302 using a narrower slipping rangeinstead of conventional, broad slipping range.

FIG. 10 provides timing diagrams showing variations of a steering angle(top diagram 400), wheel slip variations and a range between maximum andminimum allowed wheel slips calculated by the engine control unit 370(middle diagram 404), and control commands for loading and/or lockingthe LSD 302 (bottom diagram 430). For ease of illustration and withoutlimiting the generality of the present disclosure, the diagrams of FIG.10 are made in view of a constant speed of the vehicle 40, with itsfront wheels 44 rotating at an average speed of 100 RPM.

The top diagram 400 illustrates a steering angle 402 showing rotationsof the steering wheel 58 between −360 and +360 degrees, over a 32-secondperiod of time. An equivalent diagram showing angular variations of thefront wheels 44, which are steered by action of the steering wheel 58 isalso contemplated, in which case lesser angular ranges would be shown.The user initially maintains the steering wheel 58 in a straight (0degree) position from an initial zero time to about 4.5 seconds. Fromthat point in time, the user turns the steering wheel to cause thevehicle 40 to make a left turn, followed by a right turn, another leftturn, and so on. A dashed line 424 highlights a starting point in timeof the effect of this action from the user on other diagrams of FIG. 10.For illustration purposes, the user fully rotates the steering wheel 58,between −360 in left turns and +360 degrees in right turns.

A middle diagram 404 illustrates wheel slip variations between the leftand right front wheels 44 over time. For illustration purposes, thediagram 404 shows a wheel slip 406 illustrated from the standpoint ofthe left front wheel. In the context of the present disclosure, the“wheel slip” is defined as a difference in the rotational speeds of thetwo (2) front wheels 44 of the vehicle 40. For illustration purposes,the wheel slip is calculated with the left wheel 44 as a reference. Assuch, a positive wheel slip value indicates that the left wheel rotatesfaster than the right wheel while a negative wheel slip value indicatesthat the left wheel rotates slower than the right wheel. When thevehicle 40 is taking a left turn for example, the left wheel naturallyrotates slower than the right wheel, assuming no actual slip between thewheel and the ground. The curve of the wheel slip 406 thereforerepresents the speed of the left wheel minus the speed of the rightwheel for different steering angles.

The vertical axis of the diagram 404 shows wheel slip values between −60and +60 RPM. Assuming there is no slipping between the wheels and theterrain, the inside left wheel rotates at a slower rate than the outsideright wheel when turning left and thus the wheel slip is negative forall steering angles between 0 and −360. When the steering wheel isturned in the opposite direction, between 0 and +360 steering angles,the left wheel rotates faster than the right wheel and thus the wheelslip is positive. Without any slipping between the wheels and theterrain, the curve for the wheel slip 406 follows an expected wheel slipthat naturally results at a turning radius of the vehicle 40, theturning radius being in turn a function of the steering angle. A largesteering angle causes the vehicle 40 to take a small turning radius, inturn causing an important wheel slip.

As illustrated, the curve for the wheel slip 406 is jagged, primarilybecause of noise in the measurements from the wheel speed sensors 376,378, which may be caused for example by the wheels 44 hitting bumps andholes on the road.

The curve for the wheel slip 406 is for a particular implementation ofthe vehicle 40 with its front wheels 44 rotating at an average of 100RPM without slipping with respect to the ground. For thisimplementation, the expected wheel slip at the maximum steering angle of+/−360 degrees is 30 RPM, with the inside front wheel 44 rotating at 85RPM while the outside front wheel 44 rotates at 115 RPM, an average ofthe speeds of the front wheels 44 being 100 RPM. Otherwise stated, inthis particular implementation, the vehicle 40 has a slip ratio of 30%,which is a fixed value defined as a ratio between the wheel slip valueat the maximum steering angle over the average wheel speed. For the samevehicle 40, with an average wheel speed of 200 RPM, the expected wheelslip at the maximum steering angle is thus 60 RPM. For another vehicle,the slip ratio may be different depending on the steering ratio withinthe steering system of that vehicle. Also, in an embodiment, anothervehicle may have a steering that can rotate by more or less than +/−360degrees. For example, a steering wheel could be turned by more than onefull turn to steer the wheels.

The diagram 404 also shows a maximum allowed wheel slip 408 and aminimum allowed wheel slip 410. Generally speaking, the maximum allowedwheel slip 408 has a peak value when the left wheel is on the outside ofa curve (right turn) while the minimum allowed wheel slip 410 has a peak(negative) value when the left wheel is on the inside of a curve (leftturn). Together, the maximum and minimum allowed wheel slips 408 and 410define, for a given steering angle, a permissible slipping range for thefront wheels 44. The wheel slip 406 may vary between these values beforeintervention from the control unit 370 to start loading the LSD 302.

The control unit 370 uses steering angle information from the steeringangle sensor 390 to control the limited slip differential assembly 300.The control unit 370 determines the expected wheel slip that naturallyresults at a turning radius of the vehicle 40, the turning radius beingin turn a function of the steering angle. The control unit 370 adds andsubtracts a slip margin to and from the expected wheel slip,respectively, in order to expand the permissible range of relative slipbetween the front wheels 44. The slip margin may be fixed. The wheelslip may alternatively vary according to the rotational speed of thefront wheels 44. The use of a slip margin prevents excessive reaction ofthe limited slip differential assembly 300 when a rotational speeddifference of the front wheels 44 is within the permissible slippingrange. The wheel slip margin is determined by the control unit 370. Inan implementation, the slip margin may be selected at least in part sothat noise from the measurements by the wheel speed sensors 376, 378does not cause accidental interaction of the LSD 302. In theillustration of FIG. 10, the same slip margin is used for determiningthe maximum and minimum allowed wheel slips 408 and 410. Using differentslip margins for any given steering angle and/or for determining rangesof allowable wheel slips for inside and outside wheels 44 is alsocontemplated. To calculate the maximum allowed wheel slip 408, the slipmargin is added to the expected wheel slip at the current steeringangle, for a given rotational speed of the front wheels 44. To calculatethe minimum allowed wheel slip 410, the slip margin is subtracted fromthe expected wheel slip at the current steering angle, for a givenrotational speed of the front wheels 44.

FIG. 11 is a graphical representation of a slip margin (top diagram 418)varying as a function of the rotational speed for the front wheels 44 ofthe vehicle 40 (bottom diagram 414). A bottom diagram 414 shows a speed416 of the front wheels 44 of the vehicle 40, in RPM. A top diagram 418shows a slip margin 420 for the wheels 44 and a noise level 422 from themeasurements of the wheel speed sensors 376, 378. As the speed 416 ofthe vehicle increases, the noise level 422 increases as well. For thatreason, the slip margin 420 used in the determination of the maximum andminimum allowed wheel slips 408 and 410 depends at least in part on thespeed 416 of the vehicle so that the slip margin 420 remains greaterthan the noise level 422 in most circumstances. A relationship betweenthe slip margin 420 and the speed 416 of the front wheels 44 may belinear or non-linear. In an implementation, a slip margin 420 of 25 RPMcorresponds to an average speed of the front wheels 44 equal to 100 RPM.In an implementation, the control unit 370 stores a slip mapping table(sometimes called a look up table) of the relations between values ofthe slip margin 420 and speed 416 of the front wheels 44. A relationshipbetween the steering angle and the angle of the steered wheels may belinear or non-linear. The ratio of the steering wheel input to the angleof the steered wheels is however known at all times by a controller ofthe drive-by-wire steering system.

Returning now to FIG. 10, the control unit 370 determines the speed ofthe front wheels 44 by averaging measurements of the wheel speed sensors376, 378. A measurement of the steering angle is provided to the controlunit 370 by the steering angle sensor 390. Before about 4.5 seconds(dashed line 424), the steering wheel 58 is held in a straight positionand the expected wheel slip is zero RPM. The maximum and minimum allowedwheel slips 408, 410 are at the time respectively equal to the 25 RPMslip margin above and below the expected wheel slip value, this slipmargin being for the front wheels 44 rotating at 100 RPM on average. At4.5 seconds, the user starts turning the steering wheel 58, at first tothe left and then to the right, and so on. The control unit 370 usesmeasurements from the steering wheel angle sensor 390 to modify themaximum and minimum allowed wheel slips 408 and 410 that may be allowedbefore applying a load to the LSD 302. In the illustrated example, atabout 11.5 seconds (dashed line 426), the steering wheel 58 is turned tothe right by 360 degrees, which causes a 30 RPM difference between thespeeds of the front wheels 44, given the current wheel speed of 100 RPMand the 30% slip ratio of the vehicle 40. The left wheel 44 on theoutside of the curve rotates at a higher speed while the right wheel 44on the inside of the curve rotates at a lower speed (generally at point428 on the wheel slip 406 curve). At that time, the minimum allowedwheel slip 410 is +5 RPM (30-25 RPM) while the maximum allowed wheelslip 408 is +55 RPM (30+25 RPM). Otherwise stated, the left wheel 44being at the time the outside wheel would naturally rotate faster thanthe inside right wheel 44 by 30 RPM if on non-slippery terrain. Giventhe permissible slipping range, the left wheel 44 is allowed to rotateeven faster, up to 55 RPM faster than the inside right wheel 44, beforethe control unit 370 starts applying a load to the LSD 302. At the sametime, the positive value of the minimum allowed wheel slip 410 impliesthat the control unit 370 will apply a load to the LSD 302 if theoutside left wheel 44 rotates less than 5 RPM faster than the insideright wheel 44. The 5 RPM value is calculated as the expected wheel slipat the current angle of the steering device (30 RPM) minus the slipmargin, which has a value of 25 RPM. The difference between the maximumallowed wheel slip 408 and the minimum allowed wheel slip 410 ismaintained constant at 50 RPM, this value reflecting the slip margin of25 RPM being applied on both sides of the expected wheel slip, for the100 RPM wheel speed.

A lower diagram 430 of FIG. 10 shows commands from the control unit 370to load and then unload the LSD 302. These commands are generated by thecontrol unit 360 when the wheel slip 406 moves out of the bounds definedby the maximum and minimum allowed wheel slips 408 and 410. The LSD 302is initially unloaded (command is OFF). In the present example, thewheel slip 406 exceeds the minimum allowed wheel slip 410 at about 15.5seconds and, in response, the control unit 370 sends a control commandto the LSD 302 at a 16-second mark (dashed line 432; command is ON). Thecontrol unit 370 initially causes a load to be applied to the LSD 302 byenergizing the electric motor 288. The control unit 370 may furtherenergize the solenoid 382 to lock the LSD 302. In an implementation, thecontrol unit 370 may determine a level of the load to be applied to theLSD 302 based on one or more of a plurality of parameters, includingwithout limitation a torque provided by the engine 62, a position of theshifter 60 selecting a gear ratio of the transaxle 66, a magnitude ofthe wheel slip 406, and a magnitude of an excess of the wheel slip 406in relation to the maximum or minimum allowed wheel slips 408, 410. Thecontrol unit 370 may also determine whether or not to lock the LSD 302based on a combination of these parameters.

As illustrated, starting at the 16-second mark, the LSD 302 issufficiently loaded, possibly being locked, to cause the wheel slip 406to reduce substantially to zero RPM. At the same time, the control unit370 adapts its calculation of the maximum and minimum allowed wheelslips 408 and 410. Before detecting that the wheel slip 406 is movingout of the bounds defined by the maximum and minimum allowed wheel slips408 and 410, the maximum allowed wheel slip 408 calculated according tothe steering angle is at −5 RPM (point 429) and the minimum allowedwheel slip 410 calculated according to the steering angle is at −55 RPM.The LSD 302 is loaded, and possibly locked, by the control unit 370. Theactual wheel slip is thus reduced substantially to zero RPM.

Assuming that the control unit 370 would still determine the maximumallowed wheel slip 408 based on the steering angle, in the manner asdescribed earlier, the maximum allowed wheel slip 408 would be equal to−5 RPM at that time and the control unit 370 would control theapplication of a load to the LSD 302 because of the zero RPM wheel slipbeing greater than −5 RPM. The LSD 302 being already loaded, this actionof the control unit 370 would be superfluous. Consequently, the controlunit 370 modifies its calculation of the maximum allowed wheel slip 408in the manner expressed hereinbelow. At the same time, the control unit370 would not act upon the minimum allowed wheel slip 410 calculated inview of the steering angle because, at −55 RPM, this minimum allowedwheel slip would not be exceeded. There is no need to modify thecalculation of the minimum allowed wheel slip 410 at that time.

When the LSD 302 is loaded, the control unit 370 updates the maximumallowed wheel slip 408 by selecting the greater of: (a) a sum of theexpected wheel slip and the slip margin for the current wheel speed; and(b) the slip margin for the current wheel speed. In the present example,as shown on the diagram 404, the maximum allowed wheel slip 408 becomesequal to the slip margin starting at the 16-second mark. The controlunit also updates the minimum allowed wheel slip 410 by selecting thelower (most negative) of: (a) the expected wheel slip minus the slipmargin for the current wheel speed; and (b) the slip margin for thecurrent wheel speed expressed in the negative (i.e. zero minus the slipmargin). In the present example, in that case, the minimum allowed wheelslip 410 remains unchanged because it is lower than the slip marginexpressed in the negative. As a result, the maximum allowed wheel slip408 is changed by the control unit 370 to 25 RPM (0+25 RPM), this valueof the maximum allowed wheel slip 408 being the same as when thesteering wheel 58 is held in a straight position (zero steering angle).Without this calculation change, the curve of the maximum allowed wheelslip 408 could intersect the actual wheel slip 406, which issubstantially zero RPM at the time. In the example as illustrated onFIG. 10, the minimum allowed wheel slip 410 continues being calculatedbased on the actual angle of the steering wheel 58 and no intersectiontakes place between the wheel slip 406 and the minimum allowed wheelslip 410.

From the 16-second mark (dashed line 432), the control unit 370 tracksthe wheel slip 406 and may gradually increase or decrease the load tothe LSD 302 depending on a synchronization of the front wheels 44 andpossibly depending on some of the above mentioned parameters used by thecontrol unit 370 to determine the level of the load to be applied to theLSD 302. At 18 seconds (dashed line 434), the wheel slip 406 is withinthe maximum and minimum allowed wheel slips 408 and 410, and the levelof load determined by the control unit 370 is at or near zero. Thecontrol unit 370 removes the loading command applied to the LSD 302 andrecalculates the maximum allowed wheel slip 408 using the calculationmethod used before the 16-second mark, in which the maximum and minimumallowed wheel slips 408 and 410 are calculated according to the steeringangle and to the slip margin, the latter optionally depending on therotation speed of the wheels 44.

FIG. 23 is a block diagram 1200 showing internal operations of thecontrol unit 370 for determining the maximum and minimum allowed wheelslips 408 and 410 according to an implementation. The block diagram 1200shows three (3) inputs that may be used by the control unit 370 todetermine the maximum and minimum allowed wheel slips 420 and 410. It iscontemplated that, in an implementation, this determination may be basedon additional inputs. One such input is an average rotational wheelspeed 1202 of the front wheels 44, expressed in RPM. Another input is asteering angle 1204 provided by the steering angle sensor 390, expressedin degrees. The steering angle 1204 may represent the angle of asteering wheel 58 or the angle of a handlebar. A further input is abinary status 1206 of the LSD 302. The binary status 1206 is set if theLSD 302 is loaded and/or locked. The binary status 1206 is resetotherwise.

The average rotational wheel speed 1202 is applied to the slip mappingtable, which is illustrated as a block 1208 on FIG. 28. The slip mappingtable outputs a slip margin 1210. In the example of FIG. 23, the slipmargin 1210 has a positive value applied to a first adder 1212 and to amultiplier 1214 that has a gain of −1 to produce a negative version 1216of the slip margin 1210, this negative version 1216 being applied to asecond adder 1218.

The steering angle 1204 is applied to a block 1220 that is illustratedas a graphical representation of a steering angle mapping table thatprovides a correction factor 1222 as a function of the steering angle1204. Table I is a non-limiting example of the steering angle mappingtable.

TABLE I Steering angle Correction 1204 factor (degrees) 1222 −450 0.32−360 0.23 −180 0.1  −110  0.078  −40  0.015   40 −0.015  110 −0.078  180−0.1   360 −0.23   450 −0.32 

The block 1220 outputs the correction factor 1222. Because the steeringangle 1204 may have a positive or a negative value, the correctionfactor 1222 may also have a positive or a negative value. A multiplier1224 multiplies the average rotational wheel speed 1202 by thecorrection factor 1222 to provide an expected wheel slip 1226. Theexpected wheel slip 1226 is applied to inputs of a first switch 1228 andof a second switch 1230. It may be observed that the expected wheel slip1226 may also have a positive or a negative value.

The steering angle 1204 is also applied to first and second comparators1232 and 1234. The first comparator 1232 outputs a logical value 1236,for example a logical 1, when the steering angle is less than or equalto zero degrees. The second comparator 1234 outputs a logical value1238, for example a logical 1, when the steering angle is greater thanor equal to zero degrees. The binary status 1206 of the LSD 302 isapplied to first and second AND boxes 1240 and 1242, along with,respectively, the logical values 1236 and 1238.

If the binary status 1206 of the LSD 302 is not set, the outputs of bothAND boxes are reset, for instance producing logical 0's applied to theswitches 1228 and 1230. In that case, outputs 1244 and 1246 of theswitches 1228 and 1230 are both set to the expected wheel slip 1226. Theadder 1212 sums the expected wheel slip 1226 and the slip margin 1210 toyield the maximum allowed wheel slip 408. The adder 1218 sums theexpected wheel slip 1226 and the negative version 1216 of the slipmargin 1210 to yield the minimum allowed wheel slip 410.

If the binary status 1206 of the LSD 302 is set, the LSD 302 beingloaded or locked at the time, provided that the steering angle is notequal to zero degree, one of the AND boxes 1240 or 1242 issues alogical 1. If the steering angle 1204 is negative, the logical value1236 is set and the AND box 1240 issues a logical 1 applied to theswitch 1228. The output 1244 of the switch 1228 is set to a fixed value1248, for example equal to 0 RPM. This value is added to the slip margin1210 and the maximum allowed wheel slip 408 becomes equal to the slipmargin 1210. This situation is exemplified on FIG. 10, between dashedlines 432 and 434, when the steering angle 1204 is negative and the LSD302 is locked. At the same time, the logical value 1238 from thecomparator 1234 is reset because the steering angle 1204 is not greaterthan or equal to zero. The AND box 1242 issues a logical 0 applied tothe switch 1230. The output 1246 of the switch 1230 is not changed andthe minimum wheel slip 410 remains equal to the sum of the expectedwheel slip 1226 and the negative version 1216 of the slip margin 1210.

If the binary status 1206 of the LSD 302 is set and if the steeringangle 1204 is positive, the logical value 1238 is set and the AND box1242 issues a logical 1 applied to the switch 1230. The output 1246 ofthe switch 1230 is set to a fixed value 1250, for example equal to 0RPM. This value is added to the negative version 1216 of the slip margin1210 and the minimum allowed wheel slip 410 becomes equal to thenegative version 1216 of the slip margin 1210. This situation isexemplified on FIG. 10, between dashed lines 436 and 438, when thesteering angle 1204 is positive and the LSD 302 is locked. At the sametime, the logical value 1236 from the comparator 1232 is reset becausethe steering angle 1204 is not less than or equal to zero. The AND box1240 issues a logical 0 applied to the switch 1228. The output 1244 ofthe switch 1228 is not changed and the maximum wheel slip 408 remainsequal to the sum of the expected wheel slip 1226 and the slip margin1210.

If the binary status 1206 of the LSD 302 is set and the steering angleis equal to zero degrees, the AND boxes 1240 and 1242 each issue alogical 1. Because the expected wheel slip 1226 is at or near 0 RPM atthat times, all the selectable inputs of the switches 1228 and 1230 areequivalently set to 0 RPM and thus the outputs of the AND boxes 1240 and1242 have no impact on the calculations of the maximum and minimumallowed wheel slips 408 and 410.

Returning to FIG. 10, the control unit 370 may determine the proper timeto remove the load to the LSD 302 according to a plurality ofparameters. In a variant, the control unit 370 may remove the loadingwhen the wheel slip 406 has remained within a permissible range definedby the maximum and minimum allowed wheel slips 420 and 410 for apredetermined time duration. In the same or another variant, a level ofthe loading may be gradually reduced by the control unit 370, the levelbeing based at least in part on the magnitude of the excess of thecurrent wheel slip in relation to the range between the maximum andminimum allowed wheel slips 408 and 410.

For example, FIG. 13 is a logic diagram showing details of a method ofapplying a load on the limited slip differential. In a sequence 450,operation 452 comprises monitoring signals and measurements from thewheel speed sensors 376, 378 for eventually detecting that the wheelslip 406 exceeds the permissible slipping range defined by the maximumand minimum allowed wheel slips 408 and 410. Following such detection,the control unit 370 controls a loading of the LSD 302 at operation 454.Referring again to FIG. 9, the control of the LSD 302 to reduce thecurrent wheel slip 406 is effected by loading the LSD 302. To this end,the clutch 374 of the LSD 302 is compressed to reduce a rotational speeddifference of the output shafts 322 and 324, which are operativelyconnected to the half shafts 98 and further to the left and right drivenwheels 44. Compression of the clutch 374 of the LSD 302 is made by theelectric motor 288, which rotates the gear set 276 that in turntranslates its rotational movement into an axial motion of the ball ramp278 to apply a torque TqDiff on the clutch 374.

An initial torque value Tq is applied on the clutch 374. The initialtorque value Tq may, for example, be proportional to the torque providedby the engine 62. If excessive wheel slip remains, the torque TqDiff israised by an increment Δ_up1 at operation 456. Operation 456 may berepeated until the current wheel slip 406 returns within the permissibleslipping range. Once the current wheel slip 406 has returned within thepermissible slipping range, operation 458 gradually decreases the torqueTqDiff applied on the clutch 374 by Δ_down steps. If the wheel slip 406increases and falls again outside the permissible slipping range, thetorque TqDiff is raised an increment Δ_up2 at operation 460, followingwhich the sequence returns to operation 456. The increments Δ_up1 andΔ_up2 may either have equal or unequal values. When the torque TqDifffalls to zero, the wheel slip 406 being within the permissible slippingrange, slip monitoring resumes at operations 452.

Other manners of calculating the torque TqDiff applied on the clutch 374are contemplated. The torque may for example be calculated proportionalto a difference between the current wheel slip 406 and the maximum andminimum allowed wheel slips 408 and 410.

Returning to FIG. 10, the wheel slip 406 is once again out of bounds at20 seconds (dashed line 436), with the steering wheel 58 now turned tothe right. The control unit 370 sends a control command to load the LSD302. As a result, the LSD 302 becomes sufficiently loaded, and possiblylocked, to reduce the wheel slip 406 substantially to zero RPM. At thesame time, the control unit 370 adapts its calculation of the maximumand minimum allowed wheel slips 408 and 410. Before detecting that thewheel slip 406 is moving out of the bounds defined by the maximum andminimum allowed wheel slips 408 and 410, the maximum allowed wheel slip408 calculated according to the steering angle is at about +55 RPM andthe minimum allowed wheel slip 410 calculated according to the steeringangle is at about +5 RPM (point 437). At the 20-second mark, the controlunit 370 updates the maximum allowed wheel slip 408 by selecting thegreater of: (a) a sum of the expected wheel slip and the slip margin forthe current wheel speed; and (b) the slip margin for the current wheelspeed. In the present example, as shown on the diagram 404, the maximumallowed wheel slip 408 remains unchanged as it is greater than the slipmargin. The control unit also updates the minimum allowed wheel slip 410by selecting the lower (most negative) of: (a) the expected wheel slipminus the slip margin for the current wheel speed (b) zero minus theslip margin for the current wheel speed. In the present example, theminimum allowed wheel slip 410 becomes equal to the slip marginexpressed in the negative starting at the 20-second mark. As a result,the minimum allowed wheel slip 410 is changed by the control unit 370 to−25 RPM (0-25 RPM), as when the steering wheel 58 is held in a straightposition (zero steering angle). Without this calculation change, thecurve of the minimum allowed wheel slip 410 could intersect the actualwheel slip 406, which is substantially zero RPM at the time. In theexample of FIG. 10, the maximum allowed wheel slip 408 continues beingcalculated based on the actual angle of the steering wheel 58 and nointersection takes place between the wheel slip 406 and the maximumallowed wheel slip 408.

From the 20-second mark (dashed line 436), the control unit tracks thewheel slip 406 and may gradually release the load to the LSD 302. At22.5 seconds (dashed line 438), the wheel slip 406 is within the maximumand minimum allowed wheel slips 408 and 410 and the level of loaddetermined by the control unit 370 is at or near zero. The control unit370 removes the loading command applied to the LSD 302 and recalculatesthe minimum allowed wheel slip 410 according to the steering angle.

The wheel slip 406 exceeds the maximum allowed wheel slip 408 again at24.5 seconds (dashed line 440), the steering wheel 58 being turned tothe left at that time. The control unit 370 sends again a controlcommand to load the LSD 302, optionally further locking the LSD 302. Asa result, the LSD 302 becomes sufficiently loaded to substantiallyreduce the wheel slip 406 to zero RPM. At the same time, the controlunit 370 adapts its calculation of the maximum and minimum allowed wheelslips 408 and 410. Before detecting that the wheel slip 406 is movingout of the bounds defined by the maximum and minimum allowed wheel slips408 and 410, the maximum allowed wheel slip 408 calculated according tothe steering angle is at about −5 RPM and the minimum allowed wheel slip410 calculated according to the steering angle is at about −55 RPM. Thecontrol unit 370 updates the maximum allowed wheel slip 408 by selectingthe greater of: (a) a sum of the expected wheel slip and the slip marginfor the current wheel speed; and (b) the slip margin for the currentwheel speed. In the present example, the maximum allowed wheel slip 408becomes equal to the slip margin starting at the 24.5-second mark. Thecontrol unit also updates the minimum allowed wheel slip 410 byselecting the lower (most negative) of: (a) the expected wheel slipminus the slip margin for the current wheel speed; and (b) zero minusthe slip margin for the current wheel speed. In the present example, theminimum allowed wheel slip 410 remains unchanged at the 24.5-second markas it is lower than the slip margin expressed in the negative. As aresult, the maximum allowed wheel slip 408 is changed by the controlunit 370 to 25 RPM (0+25 RPM), as when the steering wheel 58 is held ina straight position (zero steering angle). The minimum allowed wheelslip 410 continues being calculated based on the actual angle of thesteering wheel 58.

The user continues turning the steering wheel 58. At about 27 seconds(dashed line 442), the steering wheel 58 is turned to the right and theleft wheel becomes the outside wheel. The control unit 370 continuesselecting the maximum allowed wheel slip 408 as the greater of themaximum allowed wheel slip calculated according to the current steeringangle and the slip margin. At the 27-second mark, the maximum allowedwheel slip 408 starts becoming greater than the slip margin. The controlunit 370 also continues selecting the minimum allowed wheel slip 410 asthe lesser (most negative) of: (a) the minimum allowed wheel slipcalculated according to the current steering angle; and (b) the slipmargin expressed in the negative. In that case, the minimum allowedwheel slip 410 becomes equal to the slip margin expressed in thenegative starting at the 27-second mark.

The control unit 370 may issue a command to load the LSD 302 in responseto other situations or driving conditions of the vehicle, for instancein response to a user command to lock the LSD 302. The LSD 302 may thusbe loaded even though at the time the wheel slip 406 may be within therange between the maximum and minimum allowed wheel slips 408 and 410.Notwithstanding the reason for loading the LSD 302, in animplementation, the control unit 370 may select the maximum allowedwheel slip 408 as the greater of: (a) a sum of the expected wheel slipand the slip margin for the current wheel speed; and (b) the slipmargin, also selecting the minimum allowed wheel slip 410 as the lesser(most negative) of: (a) the expected wheel slip minus the slip marginfor the current wheel speed; and (b) zero minus the slip margin for thecurrent wheel speed, whenever the LSD 302 is loaded.

Considering the middle diagram 404, it may be observed that aconventional limited slip differential not configured to react to thesteering angle of the vehicle 40 and configured to allow a fixed wheelslip margin between +55 RPM and −55 RPM values (dotted lines 444 and446, respectively) would operate in the following manner at points 429and 437, at the 16-second mark and at the 20-second mark, respectively.In the event of a wheel slip exceeding +/−55 RPM bounds at those times,the conventional limited slip differential could be loaded and/or lockedto bring back the wheel slip to about zero RPM. However, theconventional limited slip differential would not react as does the LSD302 at the 24.5-second mark (dashed line 440) because the wheel slipwould still be within the +/−55 RPM bounds. Otherwise stated, theconventional limited slip differential would be much slower to react tothe onset of wheel slip than the present LSD 302.

It will be understood that FIG. 10 and its description relate to thewheel slip as a difference between the speed of the left wheel minus thespeed of the right wheel, leading to obtaining negative wheel slipvalues when the left wheel rotates slower than the right wheel. Thepresent technology can also be described in terms of wheel slip valuesfor the right wheel, in which case a positive wheel slip value would beobtained when the left wheel rotates slower than the right wheel. Insuch a case, the present illustration would be modified in that themiddle diagram 404 would be flipped so that the curve for the wheel slip406 moves toward positive wheel slip values when the steering angle 402moves towards negative values. The present LSD 302 and control unit 370in fact react to differences between the speeds of the left and rightdriven wheels without prioritizing any of these wheels.

FIGS. 12a and 12b are a logic diagram showing operations of a method forcontrolling the LSD 302 based on a steering angle of the vehicle 40. Asequence 500 is best understood by consideration of FIGS. 12a and 12balong with FIG. 23. The sequence 500 comprises a plurality of operationsthat may be executed in variable order, some of the operations possiblybeing executed concurrently, some of the operations being optional. Thesequence 500 may be implemented in a vehicle, for example the vehicle40. The vehicle 40 has a steering device, for example the steering wheel58. The vehicle may alternatively implement a handlebar as a steeringdevice. A limited slip differential (LSD), for example the LSD 302, isconnected to the half-shafts 98 of the vehicle 40. Left and right drivenwheels such as the front wheels 44 are operably connected to the LSD 302via the half-shafts 98.

In the sequence 500, operation 510 comprises determining rotationalspeeds of the left and right driven wheels 44. A current wheel slip 406is calculated at operation 515 as a difference between the rotationalspeeds of the left and right driven wheels 44. The wheel slip 406 mayeither be calculated as the speed of the left wheel minus the speed ofthe right wheel or as the speed of the right wheel minus the speed ofthe left wheel. Operation 520 comprises determining the averagerotational speed 1202 of the left and right driven wheels 44. Thesteering angle 1204 is determined at operation 525. Operation 530comprises applying the current steering angle 1204 to the steering anglemapping table (Table I) to obtain the correction factor 1222 andmultiplying the correction factor 1222 by the average rotational speed1202 to determine the expected wheel slip 1226. The slip margin 1210 isselected at operation 535 by applying the average rotational wheel speed1202 to the slip mapping table, which is a representation of diagrams414 and 418 in the control unit 370.

At operation 540, the control unit 370 determines whether or not the LSD302 is currently loaded, and sets or resets the binary status 1206 ofthe LSD 302 accordingly. If the binary status 1206 is not set, theswitch 1228 allows the maximum allowed wheel slip 408 to be calculatedat operation 545 by adding the slip margin 1210 to the expected wheelslip 1210 in the adder 1212. Also if the binary status 1206 is not set,the switch 1230 allows the minimum allowed wheel slip 410 to becalculated at operation 550 by subtracting the slip margin 1210 from theexpected wheel slip 1226, the adder 1218 effectively adding the expectedwheel slip 1226 to the negative version 1216 of the slip margin 1210.

If, at operation 540, the LSD 302 is loaded, the binary status 1206 isset, and one of the AND boxes 1240 and 1242 outputs a logical 1,depending on the steering angle 1204. As expressed in the foregoingdescription of FIG. 23, outputs of the AND boxes 1240 and 1242 arerespectively applied to the switches 1228 and 1230, causing the outputs1244 and 1246 of the switches 1228 and 1230 to be equal either to theexpected wheel slip 1226 or to the fixed values 1248 and 1250, which areboth equal to 0 RPM. If for example the steering angle 1204 is negative(the logical value 1236 is set) and LSD 302 is loaded, causing thebinary status 1206 to be set, the AND box 1240 issues a logical 1,causing the output 1244 of the switch 1228 to be set to 0 RPM. At thattime, because the steering angle 1204 is negative, the expected wheelslip 1226 is also negative. Consequently, when the binary status 1206 isset, the output 1244 of the switch 1228 is the greater of the expectedwheel slip 1226 or 0 RPM. This output 1244 is added to the slip margin1210 by the adder 1212. As a result, the maximum allowed wheel slip 408is calculated at operation 555 as the greater of: (a) the slip margin1210; and (b) a sum of the expected wheel slip 1226 and the slip margin1210. In an equivalent manner, if the binary status 1206 is set, theminimum allowed wheel slip 410 is calculated at operation 560 as thelesser of: (a) the slip margin 1210 expressed in the negative 1216; and(b) the expected wheel slip 1226 minus the slip margin 1210.

Operation 565 comprises detecting that the current wheel slip 406 isoutside the range between the maximum and minimum allowed wheel slips408 and 410. This detection made at operation 565 causes the setting ofthe binary status 1206, if not previously set. If not previously loaded,the LSD 302 is loaded at operation 570. Increasing the loading of theLSD 302 following the detection made at operation 565, if the LSD 302was previously loaded, is also contemplated.

Returning to FIGS. 10 and 13, operation 570 may end, for example, at the18-second mark (dashed line 434) and at the 22.5-second mark (dashedline 438), when the torque TqDiff falls to zero, at which time thebinary status 1206 of the LSD 302 may be reset by the control unit 370.

Each of the operations of the sequences 500 and 450 may be configured tobe processed by one or more processors, the one or more processors beingcoupled to a memory. In more details, FIG. 14 is a block diagram of anexemplary control unit 370. The control unit 370 comprises a processor602 operatively connected to a memory 604, an input port 606 and anoutput port 608. The processor 602 may include a plurality ofco-processors. The memory 604 may include one or more memory modules.The input port 606 may include a plurality of input modules. Likewise,the output port 608 may include one or more output modules. The inputport 606 and the output port 608 may be integrated as an input/outputmodule.

In an implementation, the input port 606 receives signals andmeasurements from the wheel speed sensors 376, 378 and the steeringangle sensor 390, and may further receive measurements form the vehiclespeed sensor 380. The output port 608 provides control commands to theactuator 372 of the LSD 302 and to the solenoid 382 for loading and/orlocking the LSD 302. The memory 604 stores configuration information forthe control of the LSD 302, including the maximum steering angle or thevehicle 40, for example +/−360 degrees, the slip ratio for the vehicle40, for example 30%, and the slip mapping table of the relations betweenvalues of the slip margin 420 and speed 416 of the front wheels 44.

In operation, the processor 602 analyses speed measurements for bothwheels driven by the LSD 302 as well as the current steering angle. Theprocessor 602 averages the speed measurements of the driven wheels 44provided by the wheel speed sensors 376, 378. Use of measurements fromthe vehicle speed sensor 380 is also contemplated. Based on the currentsteering angle, on the average rotational speed of the wheels 44 andbased on the slip ratio stored in the memory 604, the processor 602calculates an expected wheel slip for the wheels 44. The processor 602also reads the slip margin for the average rotational speed of thewheels 44 from the memory 604 and calculates the maximum and minimumallowed wheel slips by respectively adding and subtracting the slipmargin to and from the expected wheel slip.

The processor 602 also calculates a current wheel slip as a differencebetween the rotational speeds of the left and right driven wheels 44.The processor 602 may calculate a loading level that should be appliedto the LSD 302. If the wheel speed measurements show that the currentwheel slip is within the range defined by the maximum and minimumallowed wheel slips 420 and 410, the processor 602 may determine that noload is needed; however, the processor 602 may still determine that someload is to be applied to the LSD 302 for other reasons. If the processor602 determines that the wheel slip is beyond the range defined by themaximum and minimum allowed wheel slips 408 and 410, it may cause theoutput port 608 to provide a control command to the actuator 372, inturn causing the actuator 372 to start applying load to the LSD 302. Theloading level for controlling the LSD 302 may for example be calculatedas a function of a magnitude of the wheel slip or as a function of adifference between the actual wheel slip and the range defined by themaximum and minimum allowed wheel slips 408 and 410. The calculatedloading level may be part of the control command provided to theactuator 372, for example in the form of a voltage or a current appliedto the electric motor 288, this voltage or current being calculated toprovide the calculated TqDiff value to be applied on the clutch 374. Theprocessor 602 being continuously informed of the wheel speedmeasurements, the processor 602 may thus continuously recalculate theamount of load. Under some conditions, for example at maximum loading ofthe LSD 302 or when the loading is maintained for an extended period oftime, the processor 602 may cause the output port 608 to provide anothercommand to the solenoid 382 to lock the LSD 302. The processor 602 maythen remove the application of loading and/or locking to the LSD 302when the conditions that caused the loading of the LSD 302 are no longerpresent.

As the average rotational speed of the wheels 44 may constantly change,the processor 602 continuously reevaluates the expected wheel slip forthe wheels 44 and the maximum and minimum allowed wheel slips 408 and410 in view of the changing expected wheel slip. The processor mayobtain a new value of the slip margin from the memory 604 as the averagerotational speed of the wheels 44 changes.

In an implementation, while any load is applied to the LSD 302 for anyreason, the processor 602 continuously recalculates the maximum andminimum allowed wheel slips 408 and 410 so that the maximum allowedwheel slip 408 remains equal or greater than the slip margin and so thatthe minimum allowed wheel slip 410 remains equal or lower than the slipmargin expressed in the negative.

Control of the LSD 302 Based on an Engine Torque (Rock Crawling Mode)

Another aspect of the present technology provides control of the LSD 302connected to the driven wheels 44 of the vehicle 40, this control beingbased at least in part on a current output torque of the engine 62, on aposition of an accelerator control, and on an average speed of thewheels 44. The LSD 302 is preloaded in the sense that it is placed incondition for limiting an eventual slip between the two wheels 44 beforean actual slip occurrence.

In an implementation, the user of the vehicle 40 can activate thisfeature, for example by setting the toggle switch 65 (FIG. 3) to therock crawling mode. In the rock crawling mode, the control unit 370determines whether or not to apply a preload to the LSD 302, thisdetermination being based on the current output torque of the engine 62,on a current speed of the vehicle 40, and on a user demand placed on theengine 62 via the accelerator control. The user demand is detected byconsidering the position of the accelerator control, for example anaccelerator pedal 91 (FIG. 2). It is contemplated that a twistaccelerator (not shown) or a pushbutton (not shown) mounted on ahandlebar (not shown) could be used as an accelerator control.Considering for example a relatively low speed of the vehicle 40concurrent with a high output torque of the engine 62 and/or with asudden heavy actuation of the accelerator control, the user may beleading to vehicle 40 into a steep incline, for example readying thevehicle 40 for climbing on a rocky surface. Preloading the LSD 302 atthat time, in view of an engine torque that can be predicted based onthe position of the accelerator control, enhances the control of thevehicle 40 provided to the user by preventing wheel slip before itactually happens.

Table II is a non-limiting example of a loading mapping table that maybe used by the control unit 370 to control a preload of the LSD 302. Thetable shows torque values for application on the clutch 374 of the LSD302, in Newton-meters (Nm), as a function of a predicted engine torque,also in Nm, and as a function of a speed of the vehicle 40, the speedbeing expressed both in kilometers per hour (KMH) and as an average ofthe rotational speeds of the left and right wheels 44 in revolutions perminute (RPM).

TABLE II Average Wheel Speed (RPM) 0 50 100 150 200 250 300 400 VehicleSpeed (KMH) 0.0 6.7 13.3 20.0 26.7 33.3 40.0 53.3 Predicted −20 NullNull Null Null Null Null Null Null Engine 10 Null Null Null Null NullNull Null Null Torque 20 0 Null Null Null Null Null Null Null (Nm) 30100 100 0 Null Null Null Null Null 40 200 200 150 100 0 Null Null Null50 300 300 250 200 100 Null Null Null 70 400 400 400 300 200 Null NullNull 90 500 500 500 500 500 Null Null Null

For illustration purposes, assuming an overall wheel diameter of about71 centimeters, Table II provides torque values applied on the clutch374 of the LSD 302 for speeds of the vehicle 40 up to about 53.3 KMH,corresponding to a wheel speed of 400 RPM, and for various predictedengine torque values.

Also in Table II, Null values reflect that the control unit 370 does notcause the application of any preload to the LSD 302 for correspondingcombinations of predicted engine torque and of vehicle speed values.Though not shown in the particular example of Table II, the loadingmapping table may also use Null values to represent situations where theelectric motor 288 does not apply pressure to the clutch 374 while, atthe same time, the clutch 274 and the LSD 304 are locked by energizingthe solenoid 382 to lock the gear set 276. Examples of Null valuesreflecting locking the LSD 304 by use of the solenoid are introducedhereinbelow.

The control unit 370 may determine the speed of the vehicle 40 eitherbased on the measurements from the vehicle speed sensor 380 or byaveraging the measurements from the wheel speed sensors 376, 378. Thecontrol unit 370 also receives an engine torque measurement from theengine torque monitor 394. In an implementation, the engine torquemonitor 394 determines the engine output torque based on admitted airand fuel measurements and based on an ignition timing advance. Use of atorque sensor operatively connected to the engine 62 is alsocontemplated.

The control unit 370 controls a torque to be applied on the clutch 374of the LSD 302 in view of the predicted engine torque, as determinedfrom the loading mapping table of Table II. The torque value to beapplied on the clutch 374 is converted by the control unit 370 into alevel of current that should be provided to the electric motor 288 toprovide the desired preload level. This conversion is dependent oncharacteristics of the electric motor 288, of the gear set 276 and ofthe clutch 374. In a particular implementation in which the electricmotor 288 is a 12-volt DC motor, a 500 Nm torque value applied to theclutch 374 is obtained by applying a 10 amperes current to the electricmotor 288. The electric motor 288 may alternatively be a step motor, inwhich case the control unit 370 determines a number of steps sufficientto cause the electric motor 288 to apply the desired preload level tothe LSD 302. It is also contemplated that the control unit 370 may usepulse width modulation to control the electric motor 288.

In a variant of the present technology, multiple tables similar to TableII may be defined in the control unit 370. Use of distinct loadingmapping tables defined for different available gear ratios of thetransaxle 66, as reported to the control unit 370 by the shifterposition indicator 396 may be contemplated as well.

Without limitation, the above described shifter 60 for the transaxle 66allows the driver to select between a reverse gear, high and low forwardgears and a neutral position. For illustration purposes, the loadingmapping table of Table II may apply to the reverse gear and to the lowforward gear. When the high forward gear of the transaxle 66 is selectedby the shifter 60, a distinct loading mapping table is applied by thecontrol unit 370. Table III provides a non-limiting example of a loadingmapping table applicable when the transaxle 66 is in high gear:

TABLE III Average Wheel Speed (RPM) 0 50 100 150 200 250 300 400 VehicleSpeed (KMH) 0.0 6.7 13.3 20.0 26.7 33.3 40.0 53.3 Predicted −20 NullNull Null Null Null Null Null Null Engine 10 Null Null Null Null NullNull Null Null Torque 20 Null Null Null Null Null Null Null Null (Nm) 300 Null Null Null Null Null Null Null 40 100 100 0 Null Null Null NullNull 50 200 200 150 100 0 Null Null Null 70 300 300 250 200 100 NullNull Null 90 400 400 400 300 200 Null Null Null

When the transmission is in high gear, for a given predicted enginetorque and a given vehicle speed, the torque applied on the clutch 374of the LSD 302 is lower than when the transmission is in low gear, withthe predicted engine torque and the same vehicle speed. In thenon-limitative example of Table III, when the transaxle 66 is in highgear, torque values applied on the clutch 374 of the LSD 302 as afunction of the predicted engine torque and as a function of a speed ofthe vehicle 40 are shifted toward the bottom of the loading mappingtable so that, for example, when in high gear, preloading is applied fora predicted engine torque of 90 Nm at the same level as for a predictedengine torque of 70 Nm when in low gear.

In the same or another variant of the present technology, an estimationof the desired acceleration by the user of the vehicle 40 may be made bythe control unit 370. To this end, the control unit 370 uses real-timeinformation from the accelerator control sensor 392. The acceleratorcontrol sensor 392 provides signals indicative of a current position ofthe accelerator control actuated by the user of the vehicle 40, forexample the accelerator pedal 91 (FIG. 2). It is contemplated that theaccelerator control sensor 392 may alternatively provide a position of abutterfly valve in a throttle of the engine 62. The control unit 370determines an acceleration of the accelerator control position based onthe real-time information provided by the accelerator control sensor392. A large, positive acceleration of the accelerator control positionindicates that the user wishes the vehicle 40 to accelerate rapidly. Alarge, negative acceleration (i.e. deceleration) of the acceleratorcontrol position indicates that the user intends the vehicle 40 to slowdown rapidly. It may be noted that the accelerator control sensor 392 isexpected to react more rapidly than the driveline of the vehicle 40,including the engine 62, the CVT 64, the transaxle 66, and the like. Assuch, the control 370 is able to modify the preload to the LSD 302before the actual increase or decrease of torque from the engine 62becomes present at the input shaft 90.

The preload may thus be based on a predicted engine torque calculatedaccording to equation (1):

$\begin{matrix}{{{predicted\_ engine}{\_ torque}} = {{sensed\_ torque} + {\frac{\partial{taccelerator\_ position}}{\partial t} \cdot {constant}}}} & (1)\end{matrix}$

Wherein:

-   -   predicted_engine_torque is a prediction of the engine output        torque;    -   sensed_torque is a measurement of the engine output torque;    -   accelerator_position is a measurement of the accelerator control        position;

$\frac{\partial{taccelerator\_ position}}{\partial t}$

-   -    is a rate of change of the accelerator control position over        time; and    -   constant is a constant whose value is predetermined based on        characteristics of the driveline of the vehicle 40.

Per equation (1), the predicted engine torque is therefore calculated byadding a torque adjustment, which is proportional to the rate of changeof the accelerator control position over time, to the actual engineoutput torque as measured.

In an implementation, the control unit 370 implements equation (1) tocalculate the predicted engine torque. In an alternative implementation,the control unit 370 uses a torque mapping table to store relationsbetween the predicted engine torque, the accelerator control positionand the rate of change of the accelerator control position.

FIG. 15 illustrates a block diagram showing internal operations of thecontrol unit 370 for determining the predicted engine torque, accordingto an implementation. The block diagram 610 shows that up to three (3)inputs may be used by the control unit 370 to determine the predictedengine torque. It is contemplated that, in an implementation, thecontrol unit 370 may use additional inputs to determine the predictedengine torque. One such input is a current engine output torque 612,expressed in Nm, this value being provided to the control unit 370 bythe engine torque monitor 394. Another input is a current acceleratorcontrol position 614, expressed in percentage, for example the positionof the accelerator pedal 91, this value being provided to the controlunit 370 by the accelerator control sensor 392. Yet another input is arate of change 616 of the accelerator, expressed in a percent variationof the accelerator position per second. In the implementation of FIG.15, the rate of change 616 of the accelerator is determined by thecontrol unit 370 based on a signal by the accelerator control sensor392. In a variant, the control unit 370 may internally track changes tothe accelerator control position 614 to calculate the rate of change 616of the accelerator.

The rate of change 616 of the accelerator is multiplied by apredetermined time-limiting interval 618 by a multiplier 620 to providean accelerator control gain 622. In the non-limiting example of FIG. 15,the time-limiting interval 618 is equal to 0.75 second. This valueimplies the rate of change 616 of the accelerator is considered over a0.75-second period. The accelerator control gain 622 and the acceleratorcontrol position 614 are applied to an adder 624. The adder 624 outputsan equivalent accelerator control position 626 that may be greater than100% and thus exceed a realistic complete opening. This equivalentaccelerator control position 626 is applied to a block 628. The block628 is illustrated as a graphical representation of a torque mappingtable that provides an engine torque estimate 630 as a function of theequivalent accelerator control position 626. Table IV provides anon-limiting example of a content of the torque mapping table that maybe used by the control unit 370.

TABLE IV Equivalent accelerator Engine control Torque position Estimate626 (%) 630 (Nm)   0  0   9  8.5  20  17.2  30  27.9  40  41.7  50  56.4 60  70.4  70  83.6  80 118.6 100 175   120 175  The accelerator control position 614 and the accelerator control gain622 are also applied to a selection box 632 that selects the lower ofthese two (2) inputs. The selection box 632 then outputs a minimumaccelerator control position 634, which is the lower of the acceleratorcontrol position 614 and the accelerator control gain 622. A divider 636divides the minimum accelerator control position 634 by the acceleratorcontrol position 614 to provide an accelerator control correction factor638 to be applied to the engine torque estimate 630. The acceleratorcontrol correction factor 638 is in a range between zero (0) and one(1), and is dimensionless. If both the minimum accelerator controlposition 634 and the accelerator control position 614 are equal to zero,the accelerator control correction factor 638 is set to one (1). Amultiplier 640 multiplies the engine torque estimate 630 by theaccelerator control correction factor 638 to provide a projected torque642. Because the accelerator control correction factor 638 is defined ina range between zero and one, the projected torque 642 is less than orequal to the engine torque estimate 630. Otherwise stated, theaccelerator control correction factor 638 limits the engine torqueestimate 630 in view of a realistic accelerator control positionprediction. A subtractor 644 subtracts the accelerator controlcorrection factor 638 from unity 646 (i.e. from one (1)) to produce atorque correction factor 648. A multiplier multiplies the engine outputtorque 612 by the torque correction factor 648 to produce a torquecorrection 652. The torque correction 652 is added to the projectedtorque 642 in an adder 654 to produce the desired value, which is apredicted engine torque 656.

Whether an implementation determines the predicted engine torqueaccording to equation (1) or using the operations described in relationto FIG. 15, the effect of determining the predicted engine torque issimilar. For illustration purposes, though the engine output torque mayat a given time be equal to 20 Nm for example, as reported by the enginetorque monitor 394, a detection that the user is heavily acting upon theaccelerator control may lead to the calculation of a predicted enginetorque of 90 Nm for example. Assuming that, at that time, the speed ofthe vehicle 40 is 20 KMH, the torque to be applied on the clutch 374would change from Null to 500 Nm. To control the preload, the controlunit 370 inputs the predicted engine torque in the loading mappingtables of Table II and III, one of these loading mapping tables beingselected according to the position of the shifter 60. In the aboveexample, LSD 302 would rapidly move from a no preload condition to amaximum preload condition, this result being reached much faster thanwhen the LSD 302 is loaded following the detection that the front wheels44 are slipping. Depending on characteristics of the engine 32, the CVT64, the driveshafts 82, 84 and the LSD 302 itself, the LSD 302 willreach the maximum preload condition before any actual change of a torqueat the input shaft 90 of the LSD 302. It may be noted that the rate ofchange of the accelerator control position over time may be negative ifthe user releases the acceleration control. The predicted engine torquemay therefore be lower than the engine output torque as measured,potentially causing a reduction or a release of the preload.

FIG. 16 is a sequence diagram showing operations of a method forcontrolling the LSD 302 based on an engine torque, an acceleratorcontrol position and a rate of change of the accelerator controlposition. A sequence 700 comprises a plurality of operations that may beexecuted in variable order, some of the operations possibly beingexecuted concurrently, some of the operations being optional. Thesequence 700 may be implemented in a vehicle, for example the vehicle40. The vehicle 40 has an engine and may have a transmission, forexample the engine 62 and the CVT 64 coupled to the transaxle 66. Alimited slip differential (LSD), for example the LSD 302, is operativelyconnected via the half-shafts 98 to wheels 44 driven by the engine 62.Torque from the engine 62 is applied to the LSD 302 via the input shaft90.

In the sequence 700, operation 712 comprises determining a currentoutput torque of the engine 62, this engine torque measurement beingprovided to the control unit 370 by the engine torque monitor 394. Thecontrol unit 370 determines a position of the accelerator control atoperation 714 based on a measurement from the accelerator control sensor392. A rate of change over time of the position of the acceleratorcontrol is determined at operation 716. The rate of change over time ofthe position of the accelerator control may be provided to the controlunit 370 by the accelerator control sensor 392. Alternatively, thecontrol unit 370 may calculate this rate of change based on successivemeasurements provided by the accelerator control sensor 392. The controlunit 370 determines a predicted engine torque at operation 718, thepredicted engine torque being based on the current output torque of theengine and on the rate of change over time of the position of theaccelerator control. In operation 718, the control unit 370 may add thetorque adjustment to the current output torque of the engine, accordingto equation (1). Alternatively, in operation 718, the control unit 370may implement the operations of the block diagram 610 of FIG. 15.Another alternative in which the control unit 370 applies the currentoutput torque of the engine, the position of an accelerator control andthe rate of change over time of the position of the accelerator controlto a three-dimensional look-up table to obtain the value of thepredicted engine torque is also contemplated.

Then, a speed of the vehicle 40 is determined at operation 720. Thespeed of the vehicle 40 may be provided to the control unit 370 by thevehicle speed sensor 380. Alternatively, the operation 720 may includesub-operation 722 comprising measuring, by the wheel speed sensors 376,378, rotational speeds of the left and right wheels 44. This measurementis provided to the control unit 370 that determines an average of therotational speeds of the left and right wheels 44 to determine the speedof the vehicle 40, based on this average and further based on adimension of the left and right wheels 44.

Operation 730 then comprises determining a value of a preload foreventual application to the LSD 302, the value of the preload beingbased on the predicted engine torque and on the speed of the vehicle 40.In an implementation, the operation 730 may comprise sub-operation 732in which the predicted engine torque and the speed of the vehicle 40 areapplied by the control unit 370 to a loading mapping table to readtherefrom a value of a torque to be applied on the clutch 374 of the LSD302. In a variant, the control unit may select the loading mapping tableamong a plurality of loading mapping tables, for example thoseillustrated in Tables II and III, according to a position of the shifter60, this position being provided to the control unit 370 by the shifterposition indicator 396.

Considering the loading mapping tables illustrated in Tables II and III,the value of the preload for application to the LSD 302, expressed inthe form of a torque to be applied on the clutch 374 in the presentexample, may be zero (or Null) or may be greater than or equal to zero.At operation 740, the control unit 370 conditionally causes theapplication of a preload to the LSD 302, this application beingconditional to the value of the preload being greater than zero (i.e.not Null or negative). Operation 740 may comprise sub-operation 742, inwhich the control unit 370 controls the electric motor 288 to drive thegear set 276 and the ball ramp 278 to compress the clutch 374 accordingto the torque value obtained from the loading mapping table. Thisoperation loads the LSD 302, thereby preventing or reducing a rotationalspeed difference of the left and right wheels 44.

Optionally, the gear set 276 may be locked at operation 750. To thisend, the control unit 370 may use the solenoid 382 to lock the gear set276.

Each of the operations of the sequence 700 may be configured to beprocessed by one or more processors, the one or more processors beingcoupled to a memory, for example the processor 602 and the memory 604 ofthe control unit illustrated in FIG. 14.

In an implementation, the input port 606 receives signals andmeasurements from the wheel speed sensors 376, 378, the vehicle speedsensor 380, the accelerator control sensor 392, the engine torquemonitor 394 and the shifter position indicator 396. The output port 608provides commands to the actuator 372 of the LSD 302 and to the solenoid382 for preloading and/or locking the LSD 302. The memory 604 storesconfiguration information for the control of the LSD 302, including forexample a loading mapping table or a plurality of such loading mappingtables for a plurality of positions of the shifter 60, a torque mappingtable of an engine torque estimate as a function of an acceleratorcontrol position, and dimensions of the wheels 44 expressed as a radius,a diameter or as a circumference.

In operation, the processor 602 analyses measurements obtained from thevehicle speed sensor 380 and/or measurements from the wheel speedsensors 376, 378 to determine a speed of the vehicle 40. In animplementation using the measurements from the wheel speed sensors 376,378, the control unit determines the speed of the vehicle 40 based on anaverage of the rotational speeds of the left and right wheels 44. Theprocessor 602 also analyses measurements from the engine torque monitor394. The processor 602 determines, based on the measurements from theengine torque monitor 394, a value of an output torque of the engine 62.Based on the value of the output torque of the engine 62 and on thespeed of the vehicle 40, the processor 602 determines a value of apreload, if any, to be applied to the LSD 302. The processor 602 causesthe output port 608 to provide a command to the actuator 372, in turncausing the actuator 372 to apply the preload to the LSD 302 bycompressing the clutch 374.

The configuration information stored in the memory 604 may include theloading mapping table of Table II. In an implementation, the memory 604may optionally store a plurality of loading mapping tables for each of aplurality of positions of the shifter 60, as shown for example in TablesII and III. The processor 602 may use an indication received at theinput port 606 from the shifter position indicator 396 to select one ofthese loading mapping tables in the memory 604. In any case, theprocessor 602 may apply the value of the output torque of the engine 62and the speed of the vehicle 40 to the loading mapping table todetermine a torque value to be applied on the clutch 374 of the LSD 302.The configuration information stored in the memory 604 may also includethe torque mapping table of Table IV.

The processor 602 is optionally informed, via the input port 606, of acurrent position of the accelerator control provided by the acceleratorcontrol sensor 392. The accelerator control sensor 392 may also providea rate of change of the accelerator control position, or the processor602 may continuously determine a rate of change of the acceleratorcontrol position based on successive signals from the acceleratorcontrol sensor 392. If the processor 602 determines that the user hasrapidly increased or decreased the accelerator control demand, theprocessor 602 may, in an implementation, apply the correction factor ofequation (1) to obtain a prediction of the engine output torque of theengine 62. In another implementation, the processor 602 may implementthe elements of the block diagram 610 and execute its various operationsto determine the prediction of the engine output torque of the engine62. In any case, this predicted output torque is used as the value ofthe output torque of the engine 62 for reading torque value to beapplied on the clutch 374 of the LSD 302 from the loading mapping table,in view of preloading for the LSD 302.

The processor 602 may cause the output port 608 to provide a command toenergize the solenoid 382, thereby causing the tooth 384 of the solenoid382 to engage the gear set 276, effectively locking the LSD 302. Thiscommand to lock the LSD 302 may follow the application of a preload, forexample a maximum compression of the clutch 374.

Control of the LSD 302 Optimized for Slippery Driving Conditions (MudMode)

A further aspect of the present technology provides control of the LSD302 connected to the driven wheels 44 of the vehicle 40, this controlbeing based at least in part on a detection of slippery drivingconditions such as, for example, when the vehicle 40 is in deep mud oron other poorly tractable terrain. In the context of the presentdisclosure, the term “slippery driving conditions” is used forconvenience purposes to refer to conditions such as driving on deep mudor on other very slippery surfaces, including without limitation loosegravel, icy roads, deep snow, shallow rivers, and the like. The presenttechnology is applicable, in particular but not exclusively, to off-roadvehicles.

In an implementation, the user of the vehicle 40 can activate thisfeature, for example by setting the toggle switch 65 (FIG. 3) to the mudmode. The control unit 370 can detect or infer that the vehicle 40 isrolling in deep mud or in other slippery driving condition using any oneor a combination of three (3) situations. A first situation is when apredetermined number of wheel slip occurrences is detected. A secondsituation is when a wheel is slipping despite the application of apreload to the LSD 302. A third situation is when the torque output ofthe engine is above an engine load line.

These three (3) situations are exemplified in FIG. 17, which is a blockdiagram 800 showing internal operations of the control unit 370 forcontrolling the LSD 302 in mud mode, according to an implementation. Theblock diagram 800 shows that, in the present implementation, up to four(4) inputs may be used by the control unit 370 to control the LSD 302 inthe mud mode. It is contemplated that, in an implementation, the controlunit 370 may use additional inputs.

One such input is a wheel slip detection 802. The wheel slip detection802 may be internally generated by the control unit 370 when adifference between measurements of the rotational speeds of the left andright front wheels 44 from the wheel speed sensors 376, 378 indicatesthat one of the front wheels 44 is slipping by more than a predeterminedthreshold. Before detecting a wheel slip, the control unit 370 may allowa rotational speed difference exceeding a predetermined maximumdifference in rotational speeds in order to account for the naturalspeed difference of the wheels 44 when the vehicle 40 is in a curve. Forexample in an implementation where the control of the LSD 302 based on asteering angle of the vehicle 40 and the control of the LSD 302optimized for slippery driving conditions (Mud Mode) are both supportedby the control unit 370, the wheel slip detection 802 may be generatedwhen the wheel slip 406 is out of the permissible slipping range definedby the maximum and minimum allowed wheel slips 408 and 410.

A second input is an activity indication 804 for the LSD 302, thisactivity indication 804 being set when a load is being applied to theLSD 302 through the electric motor 288. A third input is a vehicle speed806 that may be provided to the control unit 370 by the vehicle speedsensor 380. Alternatively, the vehicle speed 666 may equivalently bebased on measurements, by the wheel speed sensors 376, 378 of rotationalspeeds of the left and right wheels 44, the vehicle speed 666 beingdeduced by the control unit 370 based on these measurements and ondimensions of the front wheels 44. A fourth input is a current engineoutput torque 808, expressed in Nm, this value being provided to thecontrol unit 370 by the engine torque monitor 394.

An example will now be described with reference to FIG. 17. The blockdiagram 800 includes a number of logical components that implementlogical AND, OR and NOT functions. The present description is made withmentions of logical 1's and 0's at the output of some of the components.This manner of describing the block diagram 800 is for simplificationpurposes and does not limit the present disclosure. Otherimplementations using opposite 0's and 1's or using other logicalvalues, such as True or False, and implementations of the logic eventsof the block diagram 800 using software code, are also contemplated.

Considering the first situation, the wheel slip detection 802 is set to1 if a wheel slip is detected, or to 0 in the absence of a wheel slip.This output is applied at an input of a change determination box 810that briefly outputs a change indication 812 set to 1 when the inputchanges from 0 to 1, that is, when a wheel slip is first detected. Thechange indication 812 returns to 0 after a short delay and remains at 0if the wheel slip detection 802 is changed to 0. The change indication812 is applied to a counter 814 that counts a number of occurrences ofthe change indications 812 being set to 1. In effect, the counter 814counts a number of occurrences of distinct wheel slip events. When thecounter 814 exceeds a predetermined number of wheel slip occurrences,for example three (3) wheel slip occurrences, it applies a logical 1 toan input of a first OR box 816 having two (2) inputs. Then, regardlessof a value applied at the other input of the first OR box 816, the firstOR box 816 applies a logical 1 to an input of a second OR box 818 havingtwo (2) inputs. Then, regardless of a value applied at the other inputof the second OR box 818, the second OR box 816 issues a locking request820 for the LSD 302. The locking request 820 may cause the applicationof a load to the LSD 302, for example the application of a maximumtorque on the clutch 374, and may further cause to energize the solenoid382 to effectively lock the LSD 302. The first situation is realized byrepetitive wheel slips causing the locking of the LSD 302.

A reset box 846, which is described in details hereinbelow, may causeresetting of the counter 814 when the torque of the engine output torque808 falls to a low torque set-point 842. The counter 814 is thereforeexpected to be reset at various times during normal operation of thevehicle 40. Use of a timer to reset the counter 814 when no wheel slipdetection 802 is applied to the change determination box 810 after apredetermined time threshold is also contemplated.

Considering now the second situation, the activity indication 804 forthe LSD 302, which is set when a load is applied to the LSD 302, and thechange indication 812 are both applied to inputs of an AND box 822. TheAND box 822 outputs a 1 when both of its inputs are set to 1, which isthe case when the change indication 812 indicates that a wheel slip isdetected while the activity indication 804 indicates that a load isalready applied to the LSD 302. The 1 that is output from the AND box822 propagates through the first and second OR boxes 816, 818, thelatter issuing a locking request 820 for the LSD 302. The secondsituation is realized by the occurrence of a wheel slip while the LSD302 is loaded causing the locking of the LSD 302.

Considering now the third situation, the vehicle speed 806 is applied toa load line mapping table 824. FIG. 18 is a graph 830 of an engine loadline 832. On the graph 830 shown in FIG. 18, the load line 832 is anidealized representation of an expected engine output torque 834required to move the vehicle as a function of the vehicle speed 806 whenthe vehicle 40 travels along a flat level surface, with minimal externalresistance. On the graph 830, a point 836 represents a situation wherethe vehicle 40 is rolling in deep mud or under another slippery drivingcondition. In that situation, an actual engine torque 836, is greaterthan the expected engine output torque 834 for a given speed 836 _(S) ofthe vehicle 40.

Table V is a non-limiting example of a load line mapping table 824 thatmay be used by the control unit 370 to determine when the torquerequested by the user of the vehicle 40 is above the engine load line.The table shows the vehicle speed 806, expressed as an average of therotational speeds of the left and right wheels 44 in revolutions perminute (RPM), as a function of the engine output torque 808, inNewton-meters (Nm).

TABLE V Vehicle Speed Expected 806 Engine (front Output wheels Torque 44(RPM)) 834 (Nm)   0  20  20  20  50  20 100  20 150  25 200  30 300  40400 120 500 200 800 200

The load line mapping table 824 outputs the expected engine outputtorque 834 as a function of the vehicle speed 806. Returning to FIG. 17,the engine output torque 808 is compared with the expected engine outputtorque 834 by a first comparator 838. If the engine output torque 808 isgreater than the expected engine output torque 834, the torque demand bythe user is above the engine load line and the first comparator 838outputs a 1 that is applied to the second OR box 818. The second OR box818 issues a locking request 820 for the LSD 302. The third situation isrealized by the excess engine torque causing the locking of the LSD 302.

When none of the inputs applied to the first and second OR boxes 816 and818 is set to 1, their outputs are set to 0 and the locking request 820is not issued.

Other components of the block diagram 800 are used to remove the effectof the locking request 820, if it has already been issued.

The engine output torque 808 is compared by a second comparator 840 to alow torque set-point 842, which is set to 5 Nm in the non-limitingexample of FIG. 17. The second comparator 840 issues low torqueindication 844 set to 1 if the engine output torque 808 is less than thelow torque set-point 842. The low torque indication 844, if set to 1,causes a reset box 846 to reset the counter 814 to zero wheel slipoccurrences. As a result, the counter 814 applies a logical 0 to thefirst OR box 816. If no logical 1 is applied to other inputs of the ORboxes 816 and 818 at that time, this may lead to the removal of thelocking request 820. It may be noted that the removal of the lockingrequest 820 is not sufficient to cause the unlocking of the LSD 302 asother operating conditions of the vehicle 40 may require that the LSD302 remain locked.

The low torque indication 844 is also applied to a reset (R) input of aflip-flop box 848. The low torque indication 844 in fact applies alogical 1 to the R input of the flip-flop box 848 when the engine outputtorque 808 is lower than the low torque set-point 842.

The output of the first OR box 816 is also applied to a set (S) input ofthe flip-flop box 848. A logical 1 is therefore applied to the S inputof the flip-flop box 848 when either of the first and second situationsapplies, that is when either the predetermined number of wheel slipoccurrences is detected or when a wheel 44 is slipping despite theapplication of a preload to the LSD 302. The logical 1 is also appliedto the S input of the flip-flop box 848 when both of these situationsapply concurrently. The flip-flop box 848 has an output (Q) 850 that isset to 1 when the S input is set to 1. After being set to 1, the Qoutput 850 remains set if the S input is set to 0, until the R input ofthe flip-flop box 848 is set to 1, following which the Q output 850 isset to 0. Both S and R inputs are not expected to be set to 1 at thesame time, as this condition would require slipping of the front wheels44 while the engine torque 808 is very low. Regardless, the Q output 850is set to 0 if this circumstance occurs. Summarily, the Q output 850 isset to 1 when conditions defined in the first and second situations forlocking the LSD 302 are present.

The output of the first comparator 838 is negated by a NOT box 852. Itsoutput is a load line indication 854 set to 0 when the torque demand bythe user is above the engine load line and to 1 otherwise. A switch 856has three (3) inputs to which the Q output 850 of the flip-flop switch848, the load line indication 854 and the low torque indication 844 areapplied. The switch 856 operates as follows.

If the Q output 850 is set to 1 (one of the first and second situationscausing the issuance of the loading request 820), the switch 856 outputsthe value of the low torque indication 844. At the time, the value ofthe low torque indication 844 is expected be set to 0 because wheel slipevents causing the setting of the S input of the flip flop box 848 aregenerally not expected to occur at very low engine torque values. If theQ output 850 is set to 1 and the low torque indication is set to 0, theswitch 856 does not issue the unlocking request 858.

If the Q output 850 is set to 0 (none of the first and second usescausing the issuance of the loading request 820) and if the load lineindication 854 is set to 1, the switch 856 outputs the unlocking request858, the torque demand by the user not exceeding the engine load line atthat time (the third situation not causing the issuance of the loadingrequest 820). If the Q output 850 is set to 0 and the load lineindication 854 is set to 0, the switch 856 does not issue the unlockingrequest 858.

FIGS. 19a to 19e provide logic diagrams showing operations of a methodfor controlling the LSD 302 based on driving conditions. A sequence 900comprises a plurality of operations that may be executed in variableorder, some of the operations possibly being executed concurrently, someof the operations being optional. The sequence 900 may be implemented ina vehicle, for example the vehicle 40. In the sequence 900, operation910 comprises determining at least one parameter indicative of a ridingcondition of the vehicle 40. Based on the at least one parameter, aslippery driving condition is detected at operation 920. In response tothe detection made at operation 930, the LSD 302 is selectively lockedat operation 930. The LSD 302 may be selectively unlocked at operation940 when the slippery driving condition is no longer detected.

Operations 910 and 920 may optionally comprise sub-sequences 950 or 960,970 or 980 (FIGS. 19b to 19e ) or a combination of these sub-sequences.

In sub-sequence 950, sub-operations 952 and 954 respectively comprisedetermining a speed of the vehicle 40 and a torque of the engine 62. Theslippery driving condition is detected at sub-operation 956 bydetermining that the torque of the engine is above a load line of theengine 62 for the speed of the vehicle 40.

In sub-sequence 960, a rotational speed of the left wheel 44 and arotational speed of the right wheel 44 are determined at sub-operation962, following which the rotational speeds of the left and right wheels44 are compared at sub-operation 964. A wheel slip occurrence isdetected at sub-operation 966 if a difference between the rotationalspeeds of the left and right wheels 44 exceeds a predetermined maximumdifference in rotational speeds. The predetermined maximum difference inrotational speeds may be set to zero (0) RPM or to a larger value.

Sub-sequence 970 comprises the sub-sequence 960 for detecting wheel slipoccurrences. The sub-sequence 960 is repeated multiple times and a countof the number of wheel slip occurrences is taken at sub-operation 972The slippery driving condition is detected at sub-operation 974 when thenumber of wheel slip occurrences exceeds a predetermined number of wheelslip occurrences, for example three (3) wheel slip occurrences.Operation 976 may eventually detect that the torque of the engine islower than a low threshold value, in which case operation 978 resets thecounter of the number of wheel slip occurrences. Optionally, the counterof the number of wheel slip occurrences may also be reset when no suchoccurrence has been detected over a predetermined period of time.

In sub-sequence 980, a preload is applied to the LSD 302 atsub-operation 982. A wheel slip occurrence is detected at sub-sequence960. The slippery driving condition is detected at sub-operation 984because the wheel slip is detected while the LSD 302 is preloaded.

Each of the operations of the sequence 900 may be configured to beprocessed by one or more processors, the one or more processors beingcoupled to a memory, for example the processor 602 and the memory 604 ofthe control unit illustrated in FIG. 14.

In an implementation, the input port 606 receives signals andmeasurements from the wheel speed sensors 376, 378, from the vehiclespeed sensor 380, and from the engine torque monitor 394. The outputport 608 provides commands to the actuator 372 of the LSD 302 and to thesolenoid 382 for loading and/or locking the LSD 302. The memory 604stores configuration information for the control of the LSD 302,including for example a predetermined maximum difference in rotationalspeeds of the wheels 44 for wheel slip detection, a predetermined numberof wheel slip occurrences for the detection of successive wheel slips,the load line mapping table 824, a wheel dimension and/or the low torqueset-point 842.

In operation, the processor 602 analyses measurements and signals fromone or more of the wheel speed sensors 376, 378, the vehicle speedsensor 380, and the engine torque monitor 394. The processor 602 detectsa slippery driving condition based on at least one parameter indicativeof a riding condition of the vehicle 40, the at least one parameterbeing received at the input port 606. The processor 602 controls, viathe output port 608, locking of the LSD 302 in response to the detectionof the slippery driving condition. In various implementations theprocessor 602 may detect the slippery driving condition using one ormore of the following techniques.

For example, the processor 602 may compare the rotational speeds of theleft and right wheels 44 reported by the wheel speed sensors 376, 378and detect a wheel slip occurrence if a difference between therotational speeds of the left and right wheels 44 exceeds thepredetermined maximum difference in rotational speeds stored in thememory 604. The processor may detect the slippery driving condition whena number of wheel slip occurrences exceeds the predetermined number ofwheel slip occurrences stored in the memory 604. As mentionedhereinabove, the counter for the number of wheel slip occurrences may bereset when the engine output torque 808 falls below a low torqueset-point 842 or, optionally, after a predetermined period of timewithout any wheel slip occurrence.

In another example, the processor 602 causes the output port 608 toforward a command for applying a preload to the LSD 302, for example inresponse to the detection of a first wheel slip occurrence or for otherreasons. The processor 602 detects a slippery driving condition when awheel slip occurrence is detected while the preload is applied to theLSD 302.

In yet another example, the processor 602 may determine a vehicle speedbased on the speed measurement received at the input port 606 from thevehicle speed sensor 380, or based on an average of the rotationalspeeds of the left and right front wheels 44 as measured by the wheelspeed sensors 376, 378, factoring the dimension of the front wheels 44to determine of the vehicle speed. The processor 602 then reads, fromload line mapping table 824 stored in the memory 604, an expected enginetorque value corresponding to the vehicle speed. The processor 602detects the slippery driving condition when the engine output torquemeasurement exceeds the expected engine torque value.

When the processor 602 has caused the LSD 302 to lock in response to thedetection of a slippery driving condition, the processor 602 mayeventually control unlocking of the LSD 302. To this end, the processor602 may detect, based on one or more readings from the various sensors,that none of the conditions for the detection of the slippery drivingcondition remains. The processor 602 may then control, via the outputport 608, the unlocking of the LSD 302. In particular, the processor 602may implement the various blocks of the block diagram 800.

Control of the LSD 302 Based on an Accelerator Control Position (TrailActive Mode)

A still further aspect of the present technology provides control of theLSD 302 connected to the driven wheels 44 of the vehicle 40, thiscontrol being based at least in part on the accelerator controlposition. Heavy actuation of the accelerator control, in what iscolloquially called a “holeshot start” may, under some conditions, causethe application of a high load to the LSD 302 in order to prevent wheelspin. In view of enhancing directional stability, a stabilization loadmay also be applied to the LSD 302 when a speed of the vehicle 40 meetsor exceeds a predetermined threshold. In at least one implementation,the high load applied to the LSD 302 upon heavy actuation of theaccelerator control may be a maximum possible load that can be providedby the electric motor 288.

In an implementation, the user of the vehicle 40 can activate thisfeature, for example by setting the toggle switch 65 (FIG. 3) to thetrail active mode. Upon heavy actuation of the accelerator control bythe user at very low vehicle speed, for example upon a standing start,the control unit 370 may cause the application of a high load to the LSD302. This is expected to prevent wheel spin before it actually occurs,or at least significantly reduce the amount of wheel spin occurring as aresult of high initial acceleration. A stabilization load is alsoselectively applied to the LSD 302 when the vehicle speed exceeds apredetermined speed threshold. This stabilization load reduces potentialslipping of the front wheels 44 at high vehicle speeds, and thusimproves the directional stability of the vehicle 40.

FIG. 20 is a block diagram showing internal operations of the controlunit 370 for controlling the LSD 302 in trail active mode, according toan implementation. A block diagram 660 shows that up to three (3) inputsmay be used by the control unit 370 to load the LSD in trail activemode. It is contemplated that, in an implementation, the control unit370 may use additional inputs to determine in the trail active mode. Onesuch input is a current accelerator position 662, expressed inpercentage, this value being provided to the control unit 370 by theaccelerator control sensor 392. A minimum actuation of the acceleratorcontrol, for example a complete release of the accelerator pedal 91, maybe expressed as a 0% value. A maximum actuation of the acceleratorcontrol, for example a complete depression of the accelerator pedal 91may be expressed as a 100% value. Another input is a current engineoutput torque 664, expressed in Nm, this value being provided to thecontrol unit 370 by the engine torque monitor 394. Yet another input isa vehicle speed 666 that may be provided to the control unit 370 by thevehicle speed sensor 380. Alternatively, the vehicle speed 666 mayequivalently be based on measurements, by the wheel speed sensors 376,378 of rotational speeds of the left and right wheels 44, the vehiclespeed 666 being deduced by the control unit 370 based on thesemeasurements and on dimensions of the front wheels 44. These inputs areapplied to various blocks within the block diagram 660.

The accelerator position 662 is applied to an accelerator positionthreshold block 668 that defines a predetermined accelerator positionthreshold that may generally be considered as indicative of a fullactuation of the accelerator control. This threshold is for example at a90% depression of the accelerator pedal 91 in the non-limiting exampleof FIG. 20. Other threshold values up to 100% as well as lower valuesare also contemplated. When the accelerator position 662 meets orexceeds the predetermined accelerator position threshold, theaccelerator position threshold block 668 issues a loading command 670applied as a first input to a switch 672.

The vehicle speed 666 is applied to a steering stabilization thresholdblock 674 that defines a predetermined speed threshold. That thresholdis at 40 KMH in the non-limiting example of FIG. 20. Higher and lowerthreshold values are also contemplated. When the vehicle speed 666 meetsor exceeds the predetermined speed threshold, the steering stabilizationthreshold block 674 issues a torque assignment 676 defining a fixedtorque value, for example 350 Nm, this value being selected so that astabilization load is applied to the LSD 302, the stabilization loadbeing less than the high load. In order to prevent overheating of theelectric motor 288, application of the stabilization load to the LSD 302be followed by a command to lock the LSD 302 by activation of thesolenoid 382, following which the electric motor 288 can bede-energized.

The engine output torque 664 and vehicle speed 666 are both applied to atrail active mapping table. The trail active mapping table isillustrated as a block 678 on FIG. 20. The block 678 provides agraphical representation of the trail active mapping table. Table VIprovides a non-limiting example of a content of a trail active mappingtable that may be used by the control unit 370 to control theapplication of a load to the LSD 302. As in the case of Tables II andIII, Table VI shows torque values for application on the clutch 374 ofthe LSD 302, in Newton-meters (Nm), as a function of the output torqueof the engine 62, also in Nm, and as a function of a speed of thevehicle 40, the speed being expressed both in kilometers per hour (KMH)and as an average of the rotational speeds of the left and right wheels44 in revolutions per minute (RPM).

TABLE VI Average Wheel Speed (RPM) 0 50 100 150 200 250 300 400 VehicleSpeed (KMH) 0.0 6.7 13.3 20.0 26.7 33.3 40.0 53.3 Engine −20 Null NullNull Null Null Null Null Null Output 10 Null Null Null Null Null NullNull Null Torque 20 Null Null Null Null Null Null Null Null (Nm) 30 NullNull Null 0 50 200 350 Null 40 Null Null Null 0 100 250 350 Null 50 NullNull Null 0 150 250 350 Null 70 Null Null 0 100 250 300 350 Null 90 NullNull 100 250 350 350 350 Null

Application of engine output torque 664 and of the vehicle speed 666 tothe trail active mapping table yields a torque value 680, which may be aNull value, the torque value 680 being output by the block 678. Thetorque value 680 is a second input to the switch 672. A third input tothe switch 672 is a predetermined, high loading torque parameter 682,having a value of 750 Nm in the present implementation.

The switch 672 operates as follows. If the loading command 670 ispresent at its first input, the switch 672 issues a distinct torqueassignment 684 having a value equal to the predetermined, high loadingtorque parameter 682. If the loading command 670 is not present at thefirst input of the switch 672, the torque assignment 684 is set to thetorque value 680, which may have a Null value.

It may be noted that, in the implementation of FIG. 20, the acceleratorposition threshold block 668 may issue the loading command 670 at anyvehicle speed, provided that the current accelerator position 662 meetsor exceeds the predetermined position threshold. For that reason, thetorque assignment 684 may be set to the 750 Nm torque value of the highloading torque parameter 682 whenever the accelerator control is fullyengaged. The effect of the loading command 670 may be ignored by thecontrol unit 370 when the control unit 370 selects the torque assignment676 due to it being a nonzero value, as is the case when the vehiclespeed 666 is at least 40 KMH, effectively negating the effect of thetorque assignment 684 and of the loading command 670. When the torqueassignment 676 is at zero because the vehicle speed is less than 40 KMH,the control unit 370 selects the torque assignment 684, which may eitherhave a zero or a nonzero value and may be equal to the 750 Nm torquevalue when the loading command 670 is present. The control unit 370causes the application on the clutch 374 of the LSD 302 of the selectedone of the torque assignments 676 or 684, if one of the torqueassignments 676 and 684 is greater than zero.

FIG. 21 is a logic diagram showing operations of a method forcontrolling the LSD 302 based on an accelerator control position. Asequence 1000 comprises a plurality of operations that may be executedin variable order, some of the operations possibly being executedconcurrently, some of the operations being optional. The sequence 1000may be implemented in a vehicle, for example the vehicle 40. The vehicle40 has an engine, for example the engine 62, the engine having athrottle operatively connected to the accelerator control, for example,the accelerator pedal 91, and a limited slip differential (LSD), forexample the LSD 302.

In the sequence 1000, operation 1010 comprises determining anaccelerator control position by the accelerator control sensor 392.Optionally, a speed of the vehicle 40 may be determined at operation1020. In an implementation, operation 1020 may comprise reading thespeed of the vehicle 40 from the vehicle speed sensor 380. In anotherimplementation, operation 1020 may comprise sub-operations 1022 and1024. In sub-operation 1022, rotational speeds of the left and rightwheels 44 are determined by the wheel speed sensors 376, 378. The speedof the vehicle 40 is then determined at sub-operation 1024 based on anaverage of the rotational speeds of the left and right wheels 44 andbased on a dimension of the left and right wheels 44.

Regardless, a high load is selectively applied to the LSD 302 atoperation 1030, provided that the accelerator control position meets orexceeds a predetermined position threshold. The predetermined positionthreshold may generally be considered as indicative of a heavy actuationof the accelerator control, for example 90%. Optionally, selectivelyapplying the high load may be conditional to the speed of the vehicle 40being less than the predetermined speed threshold, for example the 40KMH value defined by the steering stabilization threshold block 674 ofFIG. 20.

The application of the high load to the LSD 302 may optionally befollowed by the energizing of the solenoid 382 to lock the LSD 302,using the technique described in the foregoing description of FIG. 9.

In an implementation where the speed of the vehicle 40 is determined,operation 1040 may comprise releasing the high load when the speed ofthe vehicle 40 meets or exceeds the predetermined speed threshold. Inthe same or another implementation, operation 1050 may comprise applyinga stabilization load when the speed of the vehicle 40 meets or exceedsthe predetermined speed threshold. In the example of FIG. 20, thesteering stabilization threshold block 674 defines the predeterminedspeed threshold.

Regardless, when a full or partial release of the accelerator control isdetected at operation 1060, the high load is released at operation 1070.A partial release of the accelerator control may for example be detectedas soon as the accelerator control sensor 392 reports an acceleratorcontrol position that no longer meets or exceeds the predeterminedposition threshold.

While the high load is not applied to the LSD 302, the acceleratorcontrol position being less than the predetermined position threshold,the control unit 370 may, at operation 1090, apply the engine outputtorque and the speed of the vehicle to the trail active mapping table toread a value of a partial load for application to the LSD. The controlunit 370 controls the application of the partial load to the LSD atoperation 1095. The partial load applied to the LSD is in mostcircumstances lower than the high load.

The values of the predetermined position threshold and of thepredetermined speed threshold are illustrative only and do not limit thepresent disclosure.

FIG. 22 is a logic diagram showing operations of a method forcontrolling the LSD 302 to stabilize the steering of a vehicle. Asequence 1100 also comprises a plurality of operations that may beexecuted in variable order, some of the operations possibly beingexecuted concurrently, some of the operations being optional. Thesequence 1100 may also be implemented in vehicle 40.

A speed of the vehicle 40 is determined at operation 1110. As in thecase of sequence 1100, the speed of the vehicle 40 may be determinedbased on measurements provided by the vehicle speed sensor 380 or by thewheel speed sensors 376, 378. A stabilization load is selectivelyapplied to the LSD 302 at operation 1120 when the speed of the vehicle40 is greater than the predetermined speed threshold. In this manner,directional stability of the vehicle 40 is enhanced.

In an implementation, the vehicle 40 may implement the sequence 1000described hereinabove with reference to FIG. 21. In anotherimplementation, the vehicle 40 may implement the sequence 1100 describedhereinabove with reference to FIG. 22. In yet another implementation,the vehicle 40 may implement both sequences 1000 and 1100. Thesesequences may therefore be implemented independently or jointly.

Each of the operations of the sequences 1000 and 1100 may be configuredto be processed by one or more processors, the one or more processorsbeing coupled to a memory, for example the processor 602 and the memory604 of the control unit illustrated in FIG. 14.

In an implementation, the input port 606 receives signals andmeasurements from the accelerator control sensor 392 and, optionally,from the wheel speed sensors 376, 378 and/or the vehicle speed sensor380. The output port 608 provides control commands to the actuator 372of the LSD 302 and to the solenoid 382 for loading and/or locking theLSD 302. The memory 604 stores configuration information for the controlof the LSD 302, including for example a dimension of the left and rightwheels 44, the predetermined position threshold and, optionally, thepredetermined speed threshold, and/or the trail active mapping table.

In operation, the processor 602 determines the control commands forcontrolling a selective application of a high load to the LSD 302 whenthe accelerator control position indicated by the accelerator controlsensor 392 meets or exceeds the predetermined position threshold storedin the memory 604. The processor 602 may receive a measurement of thespeed of the vehicle 40 from the vehicle speed sensor 380 or determinethe speed of the vehicle 40 based on an average of the rotational speedsof the left and right wheels 44 provided by the wheel speed sensors 376,378 and based on a dimension of the left and right wheels 44.Regardless, if the speed of the vehicle 40 is available, the processor602 may control the selective application of the high load to the LSD302 when the accelerator control position indicated by the acceleratorcontrol sensor 392 meets or exceeds the predetermined position thresholdon the condition that the speed of the vehicle 40 is less than thepredetermined speed threshold stored in the memory 604. The processor602 may cause the output port 608 to stop the control command for theapplication of the high load to the LSD 302 when informed by theaccelerator control sensor 392 that the accelerator control positionfalls below the predetermined position threshold or when the vehiclespeed meets of exceeds the predetermined speed threshold.

In an implementation, the control command forwarded by the output port608 causes the actuator 372 to apply the high load may be followed byanother control command for locking of the LSD 302 by the solenoid 382.

Independently from the accelerator control position, if the speed of thevehicle 40 is known, the processor 602 may determine that the speed ofthe vehicle 40 is greater than the predetermined speed threshold storedin the memory 604. In that case, the processor 602 may cause the outputport 608 to forward a control command to the LSD 302 for the applicationof a stabilization load. The stabilization load limits, withoutpreventing, a rotational speed difference between the left and rightwheels 44 of the vehicle 40 to enhance directional stability.

The processor 602 may receive, via the input port 606, an engine outputtorque value provided by the engine torque monitor 394. The processorapplies the speed of the vehicle and the engine output torque value tothe trail active mapping table stored in the memory 604 to read a valueof a partial load for application to the LSD 302. This partial load isexpressed in terms of a torque for application on the clutch 374. If theaccelerator control position is less than the predetermined positionthreshold, the processor 602 may cause the output port 608 to forward acontrol command for controlling an application of the partial load tothe LSD 302. Referring again to FIG. 20, it may be observed that if theloading command 670 is present at the input of the switch 672, anypartial load value from the trail active mapping table is ignored by theswitch 672. It may further be observed that if the torque assignment 676has a nonzero value, any partial load value from the trail activemapping table is also ignored by the control unit 370.

The present disclosure introduces various techniques for controlling theLSD 302, these techniques being exemplified in the diagrams of FIGS.12a, 12b , 13, 15, 16, 17, 19 a-e, 20, 21 and 22. A particularimplementation of limited slip differential assembly 300 mounted in aparticular vehicle 40 may integrate any one of these techniques. Anotherimplementation may integrate all of these techniques. Yet anotherimplementation may integrate any combination of these techniques.

The method, differential assembly and vehicle implemented in accordancewith some non-limiting implementations of the present technology can berepresented as follows, presented in numbered clauses.

Clauses

[Clause 1] A method of controlling a limited slip differential (LSD) ofa vehicle, the vehicle having a steering device, the LSD, and left andright driven wheels operably connected to the LSD, the methodcomprising:

determining rotational speeds of the left and right driven wheels;

calculating a current wheel slip as a rotational speed differencebetween one of the left and right driven wheels and the other one of theleft and right driven wheels;

determining a current angle of the steering device;

calculating a maximum allowed wheel slip and a minimum allowed wheelslip as a function of the current angle of the steering device;

detecting that the current wheel slip is outside a range between themaximum and minimum allowed wheel slips; and

in response to detecting that the current wheel slip is outside therange between the maximum and minimum allowed wheel slips, loading theLSD.

[Clause 2] The method of clause 1, wherein calculating the maximumallowed wheel slip and the minimum allowed wheel slip as a function ofthe current angle of the steering device comprises:

determining an average of the rotational speeds of the left and rightdriven wheels;

calculating an expected wheel slip based on:

-   -   a ratio between the current angle of the steering device and a        maximum steering angle,    -   the average of the rotational speeds of the left and right        driven wheels, and    -   a slip ratio for the vehicle;

while the LSD is not loaded, calculating the maximum allowed wheel slipby adding a slip margin to the expected wheel slip; and

while the LSD is not loaded, calculating the minimum allowed wheel slipby subtracting the slip margin from the expected wheel slip.

[Clause 3] The method of clause 2, wherein the slip margin is selectedbased on the average of the rotational speeds of the left and rightdriven wheels.

[Clause 4] The method of clause 2 or 3, further comprising, while theLSD is loaded:

calculating the maximum allowed wheel slip as the greater of:

-   -   the slip margin, and    -   a sum of the expected wheel slip and the slip margin, and

calculating the minimum allowed wheel slip as the lesser of:

-   -   zero minus the slip margin, and    -   the expected wheel slip minus the slip margin.

[Clause 5] The method of clause 4, wherein a loading level of the LSD iscalculated based on a magnitude of an excess of the current wheel slipin relation to the range between the maximum and minimum allowed wheelslips.

[Clause 6] The method of clause 4 or 5, further comprising, afterloading the LSD in response to detecting that the current wheel slip isoutside the range between the maximum and minimum allowed wheel slips:

detecting that the current wheel slip has returned within the rangebetween the maximum and minimum allowed wheel slips; and

in response to detecting that the current wheel slip has returned withinthe range between the maximum and minimum allowed wheel slips, unloadingthe LSD.

[Clause 7] The method of clause 6, wherein unloading the LSD comprisesgradually unloading the LSD.

[Clause 8] The method of any one of clauses 1 to 4, wherein loading theLSD comprises compressing a clutch of the LSD to reduce the rotationalspeed difference between the left and right driven wheels.

[Clause 9] The method of clause 8, wherein compressing the clutch of theLSD comprises using an electric motor to drive a gear set and a ballramp to apply a torque on the clutch.

[Clause 10] The method of clause 9, wherein compressing the clutch ofthe LSD further comprises using a solenoid to lock the gear set.

[Clause 11] The method of any one of clauses 8 to 10, furthercomprising:

in response to detecting that the current wheel slip is outside therange between the maximum and minimum allowed wheel slips, calculatingan initial torque for application to the clutch;

following the application of the initial torque on the clutch:

-   -   increasing the torque applied on the clutch if the current wheel        slip is still outside the range between the maximum and minimum        allowed wheel slips;    -   decreasing the torque applied on the clutch if the current wheel        slip falls within the range between the maximum and minimum        allowed wheel slips; and    -   terminating the loading to the LSD when the torque applied on        the clutch falls to zero.

[Clause 12] A differential assembly for use in a vehicle having asteering device and left and right driven wheels, the differentialassembly comprising:

a limited slip differential (LSD) operatively connectable to adriveshaft and to the left and right driven wheels, the LSD beingadapted for transferring torque from the driveshaft to the left andright driven wheels;

a steering angle sensor operatively connectable to the steering device;

wheel speed sensors operatively connectable to the left and right drivenwheels; and

a control unit operatively connected to the LSD, to the steering anglesensor and to the wheel speed sensors, the control unit being adaptedfor:

-   -   calculating, based on measurements of rotational speeds of the        left and right driven wheels received from the wheel speed        sensors, a current wheel slip as a rotational speed difference        between one of the left and right driven wheels and the other        one of the left and right driven wheels;    -   calculating, based on a measurement received from the steering        angle sensor, a maximum allowed wheel slip and a minimum allowed        wheel slip as a function of a current angle of the steering        device;    -   detect that the current wheel slip is outside a range between        the maximum and minimum allowed wheel slips; and    -   in response to detecting that the current wheel slip is outside        the range between the maximum and minimum allowed wheel slips,        controlling loading of the LSD.

[Clause 13] The differential assembly of clause 12, further comprisingan electric motor, wherein the control unit commands the electric motorto load the LSD.

[Clause 14] The differential assembly of clause 13, wherein the LSDfurther comprises a compressible clutch and wherein the electric motorloads the LSD by applying a torque on the clutch.

[Clause 15] The differential assembly of clause 14, wherein the LSDfurther comprises a gear set and a ball ramp, the gear set being adaptedfor transferring the torque from the electric motor to the ball ramp forcompressing the clutch.

[Clause 16] The differential assembly of clause 15, further comprising asolenoid having a tooth adapted for engaging the gear set when thesolenoid is energized, wherein the control unit is further adapted forcontrolling the solenoid for locking the LSD.

[Clause 17] The differential assembly of clause 16, wherein the controlunit is further adapted for controlling the LSD to apply a maximum loadbefore locking the LSD.

[Clause 18] The differential assembly of any one of clauses 12 to 17,wherein the control unit comprises:

an input port adapted for receiving measurements from the steering anglesensor and from the wheel speed sensors;

an output port adapted for forwarding control commands to the LSD; and

a processor operatively connected to the input port and to the outputport, the processor being adapted for:

-   -   calculating the maximum and minimum allowed wheel slips;    -   calculating the current wheel slip;    -   detecting that the current wheel slip is outside the range        between the maximum and minimum allowed wheel slips; and    -   in response to detecting that the current wheel slip is outside        the range between the maximum and minimum allowed wheel slips,        causing the output port to forward a control command for loading        the LSD.

[Clause 19] The differential assembly of clause 18, wherein:

the control unit further comprises a memory storing configurationinformation for controlling the LSD;

the processor is operatively connected to the memory; and

the configuration information comprises:

-   -   a maximum steering angle for the vehicle,    -   a slip ratio for the vehicle, and    -   a slip mapping table of relations between values of the slip        margin speed values for the left and right driven wheels.

[Clause 20] The differential assembly of clause 19, wherein theprocessor is further adapted for:

determining an average of the rotational speeds of the left and rightdriven wheels;

calculating an expected wheel slip based on:

-   -   a ratio between the current angle of the steering device and a        maximum steering angle,    -   the average of the rotational speeds of the left and right        driven wheels, and    -   a slip ratio for the vehicle;

reading from the slip mapping table a slip margin for the average of therotational speeds of the left and right driven wheels;

while the LSD is not loaded, calculating the maximum allowed wheel slipby adding the slip margin to the expected wheel slip; and

while the LSD is not loaded, calculating the minimum allowed wheel slipby subtracting the slip margin from the expected wheel slip.

[Clause 21] The differential assembly of clause 20, wherein theprocessor is further adapted for, while the LSD is loaded:

calculating the maximum allowed wheel slip as the greater of:

-   -   the slip margin, and    -   a sum of the expected wheel slip and the slip margin, and

calculating the minimum allowed wheel slip as the lesser of:

-   -   zero minus the slip margin, and    -   the expected wheel slip minus the slip margin.

[Clause 22] The differential assembly of clause 21, wherein theprocessor is further adapted for:

-   -   calculating a loading level of the LSD based on a magnitude of        an excess of the current wheel slip in relation to the range        between the maximum and minimum allowed wheel slips; and

configuring the control command to load the LSD according to thecalculated loading level.

[Clause 23] The differential assembly of clause 21, wherein theprocessor is further adapted for, after causing the output port toforward the control command for loading the LSD:

cause the output port to forward a control command to increase the loadto the LSD if the current wheel slip is still outside the range betweenthe maximum and minimum allowed wheel slips; and

cause the output port to forward a control command to decrease the loadto the LSD if the current wheel slip falls within the range between themaximum and minimum allowed wheel slips.

[Clause 24] A vehicle, comprising:

a frame;

a front suspension assembly connected to the frame;

a rear suspension assembly connected to the frame;

a left driven wheel and a right driven wheel connected to one of thefront and rear suspension assemblies;

at least one other wheel connected to an other one of the front and rearsuspension assemblies;

a steering device operatively connected to one of both driven wheels andthe at least one other wheel;

an engine connected to the frame;

a transmission operatively connected to the engine for receiving torquefrom the engine;

a driveshaft operatively connected to the transmission for transferringtorque from the transmission to the left and right driven wheels; and

the differential assembly of any one of clauses 12 to 23, the LSD beingoperatively connected to the driveshaft and operatively connected to theleft and right driven wheels.

[Clause 25] The vehicle of clause 24, further comprising:

a transaxle for transferring torque from the transmission to the atleast one other wheel; and

a selector adapted for selectively operatively connecting the LSD to thedriveshaft.

Modifications and improvements to the above-described implementations ofthe present technology may become apparent to those skilled in the art.For example, it is contemplated that the LSD 302 may be mounted at therear of the vehicle 40 and operatively connected to the rear wheels 48,whether the vehicle 40 has a two-wheel drive or an all-wheel driveconfiguration. The foregoing description is intended to be exemplaryrather than limiting. The scope of the present technology is thereforeintended to be limited solely by the scope of the appended claims.

What is claimed is:
 1. A method of controlling a limited slipdifferential (LSD) of a vehicle, the vehicle having a steering device,the LSD, and left and right driven wheels operably connected to the LSD,the method comprising: determining rotational speeds of the left andright driven wheels; calculating a current wheel slip as a rotationalspeed difference between one of the left and right driven wheels and theother one of the left and right driven wheels; determining a currentangle of the steering device; calculating a maximum allowed wheel slipand a minimum allowed wheel slip as a function of the current angle ofthe steering device; detecting that the current wheel slip is outside arange between the maximum and minimum allowed wheel slips; and inresponse to detecting that the current wheel slip is outside the rangebetween the maximum and minimum allowed wheel slips, loading the LSD toreduce the rotational speed difference between the one of the left andright driven wheels and the other one of the left and right drivenwheels.
 2. The method of claim 1, wherein calculating the maximumallowed wheel slip and the minimum allowed wheel slip as a function ofthe current angle of the steering device comprises: determining anaverage of the rotational speeds of the left and right driven wheels;calculating an expected wheel slip based on: a ratio between the currentangle of the steering device and a maximum steering angle, the averageof the rotational speeds of the left and right driven wheels, and a slipratio for the vehicle; while the LSD is not loaded, calculating themaximum allowed wheel slip by adding a slip margin to the expected wheelslip; and while the LSD is not loaded, calculating the minimum allowedwheel slip by subtracting the slip margin from the expected wheel slip.3. The method of claim 2, wherein the slip margin is selected based onthe average of the rotational speeds of the left and right drivenwheels.
 4. The method of claim 2, further comprising, while the LSD isloaded: calculating the maximum allowed wheel slip as the greater of:the slip margin, and a sum of the expected wheel slip and the slipmargin; and calculating the minimum allowed wheel slip as the lesser of:zero minus the slip margin, and the expected wheel slip minus the slipmargin.
 5. The method of claim 4, wherein a loading level of the LSD iscalculated based on one of (i) the current wheel slip minus the maximumallowed wheel slip if the current wheel slip is greater than the maximumallowed wheel slip and (ii) the minimum allowed wheel slip minus thecurrent wheel slip if the current wheel slip is less than the minimumallowed wheel slip.
 6. The method of claim 4, further comprising, afterloading the LSD in response to detecting that the current wheel slip isoutside the range between the maximum and minimum allowed wheel slips:detecting that the current wheel slip has returned within the rangebetween the maximum and minimum allowed wheel slips; and in response todetecting that the current wheel slip has returned within the rangebetween the maximum and minimum allowed wheel slips, terminating theloading to the LSD.
 7. The method of claim 1, wherein loading the LSDcomprises compressing a clutch of the LSD to reduce the rotational speeddifference between the left and right driven wheels.
 8. The method ofclaim 7, further comprising: in response to detecting that the currentwheel slip is outside the range between the maximum and minimum allowedwheel slips, calculating an initial torque for application to theclutch; and following the application of the initial torque on theclutch: increasing the torque applied on the clutch if the current wheelslip is still outside the range between the maximum and minimum allowedwheel slips, and decreasing the torque applied on the clutch if thecurrent wheel slip falls within the range between the maximum andminimum allowed wheel slips.
 9. A differential assembly for use in avehicle having a steering device and left and right driven wheels, thedifferential assembly comprising: a limited slip differential (LSD)operatively connectable to a driveshaft and to the left and right drivenwheels, the LSD being adapted for transferring torque from thedriveshaft to the left and right driven wheels; a steering angle sensoroperatively connectable to the steering device; wheel speed sensorsoperatively connectable to the left and right driven wheels; and acontrol unit operatively connected to the LSD, to the steering anglesensor and to the wheel speed sensors, the control unit being adaptedfor: calculating, based on measurements of rotational speeds of the leftand right driven wheels received from the wheel speed sensors, a currentwheel slip as a rotational speed difference between one of the left andright driven wheels and the other one of the left and right drivenwheels, calculating, based on a measurement received from the steeringangle sensor, a maximum allowed wheel slip and a minimum allowed wheelslip as a function of a current angle of the steering device, detectthat the current wheel slip is outside a range between the maximum andminimum allowed wheel slips, and in response to detecting that thecurrent wheel slip is outside the range between the maximum and minimumallowed wheel slips, controlling loading of the LSD to reduce therotational speed difference between the one of the left and right drivenwheels and the other one of the left and right driven wheels.
 10. Thedifferential assembly of claim 9, further comprising an electric motor,wherein the control unit commands the electric motor to load the LSD.11. The differential assembly of claim 10, wherein the LSD furthercomprises a compressible clutch and wherein the electric motor loads theLSD by applying a torque on the clutch.
 12. The differential assembly ofclaim 11, wherein the LSD further comprises a gear set and a ball ramp,the gear set being adapted for transferring the torque from the electricmotor to the ball ramp for compressing the clutch, the differentialassembly further comprising a solenoid having a tooth adapted forengaging the gear set when the solenoid is energized, the control unitbeing further adapted for controlling the solenoid for locking the LSD.13. The differential assembly of claim 12, wherein the control unit isfurther adapted for controlling the LSD to apply a maximum load beforelocking the LSD.
 14. The differential assembly of claim 9, wherein thecontrol unit comprises: an input port adapted for receiving measurementsfrom the steering angle sensor and from the wheel speed sensors; anoutput port adapted for forwarding control commands to the LSD; and aprocessor operatively connected to the input port and to the outputport, the processor being adapted for: calculating the maximum andminimum allowed wheel slips, calculating the current wheel slip,detecting that the current wheel slip is outside the range between themaximum and minimum allowed wheel slips, and in response to detectingthat the current wheel slip is outside the range between the maximum andminimum allowed wheel slips, causing the output port to forward acontrol command for loading the LSD.
 15. The differential assembly ofclaim 14, wherein: the control unit further comprises a memory storingconfiguration information for controlling the LSD; the processor isoperatively connected to the memory; and the configuration informationcomprises: a maximum steering angle for the vehicle, a slip ratio forthe vehicle, and a slip mapping table of relations between values of theslip margin and speed values for the left and right driven wheels. 16.The differential assembly of claim 15, wherein the processor is furtheradapted for: determining an average of the rotational speeds of the leftand right driven wheels; calculating an expected wheel slip based on: aratio between the current angle of the steering device and a maximumsteering angle, the average of the rotational speeds of the left andright driven wheels, and a slip ratio for the vehicle; reading from theslip mapping table a slip margin for the average of the rotationalspeeds of the left and right driven wheels; while the LSD is not loaded,calculating the maximum allowed wheel slip by adding the slip margin tothe expected wheel slip; and while the LSD is not loaded, calculatingthe minimum allowed wheel slip by subtracting the slip margin from theexpected wheel slip.
 17. The differential assembly of claim 16, whereinthe processor is further adapted for, while the LSD is loaded:calculating the maximum allowed wheel slip as the greater of: the slipmargin, and a sum of the expected wheel slip and the slip margin; andcalculating the minimum allowed wheel slip as the lesser of: zero minusthe slip margin, and the expected wheel slip minus the slip margin. 18.The differential assembly of claim 17, wherein the processor is furtheradapted for: calculating a loading level of the LSD based on one of (i)the current wheel slip minus the maximum allowed wheel slip if thecurrent wheel slip is greater than the maximum allowed wheel slip and(ii) the minimum allowed wheel slip minus the current wheel slip if thecurrent wheel slip is less than the minimum allowed wheel slip; andconfiguring the control command to load the LSD according to thecalculated loading level.
 19. A vehicle, comprising: a frame; a frontsuspension assembly connected to the frame; a rear suspension assemblyconnected to the frame; a left driven wheel and a right driven wheelconnected to one of the front and rear suspension assemblies; at leastone other wheel connected to an other one of the front and rearsuspension assemblies; a steering device operatively connected to one ofboth driven wheels and the at least one other wheel; an engine connectedto the frame; a transmission operatively connected to the engine forreceiving torque from the engine; a driveshaft operatively connected tothe transmission for transferring torque from the transmission to theleft and right driven wheels; and the differential assembly of claim 9,the LSD being operatively connected to the driveshaft and operativelyconnected to the left and right driven wheels.
 20. The vehicle of claim19, further comprising: a transaxle for transferring torque from thetransmission to the at least one other wheel; and a selector adapted forselectively operatively connecting the LSD to the driveshaft.